Starting Kindergarten (assuming you mean the US version - 5 year old public schooling), it's quite common to have behavior issues, particularly for a student who is changing schools (as many do going into Kindergarten).
Some of this is due to children finding their way through a new environment, with new authority figures, a new social dynamic, and a lot of really cool fun things. Any change, really, will set off various behavior changes - for good and bad, often - especially at that age.
I remember my oldest son when he started Kindergarten. He was probably what you'd call defiant - in that he's not a "compliant" child. He won't automatically do what he's told if he thinks it's wrong or it's not what he'd like to do. Some of that is that he's very smart, and knew a lot more than the average 5 year old (and could think in a lot more complex ways than most); but some is just who he is.
He had quite a few behavior issues his first semester, to the point that he was put on a behavior improvement plan by the teacher (not by the school, so not that serious - but close). This surprised us as well - we thought he would be a fairly well behaved child once in school, just as you.
By the end of the first semester, though, things had ironed out (so, around Christmas). The teacher worked with him to focus on the positives of his behavior and to work on the negatives, and by the end of the semester he was consistently getting "red" to "green" days (red means good, purple bad, green starting out point - think the rainbow colors). It took four months, but he settled in, and now (going into 3rd grade) has had absolutely no problems beyond boredom.
- Talk to the teacher. Be the parent who's always talking to the teacher at pickup, "How was Y's day"? Teachers will spend more resources on children whose parents show their involvement, in my experience. You also show your daughter that you're interested. My son's teacher really helped here because once I'd asked a few times, she realized it was very important to me that we have open communication about his behavior, and so she'd give me a report each day on his results. This led to me having a lot more information for when I'd talk to him - if there was a problem I knew ahead of time, and could think about how to help him.
- Talk to your daughter. Keep it positive - strictly positive - unless she brings up something negative. Ask her how her day was. Ask her what interesting things she learned. Ask her about her friends. Let her bring up any negative issues, and then help her work out how to handle them better - but stay positive. I always asked what color his day was, and if it was purple I'd say "Aww, that's too bad", and usually he'd immediately chime in with why, but if not I would let it drop; but when it was red, I'd congratulate him and ask what good things he'd done to get the bonuses. This started to help him focus on really getting lots of positive days.
- Don't help her with her school work beyond what she asks for. This is Kindergarten. She's not learning anything academic-wise that's critical at this point: what she's learning is how to learn. She has to do that herself. Let her fail. That's very important, in my opinion: children who learn how to fail and pick themselves back up again are far better equipped for life than ones who are propped up by their parents. I don't do his homework with him; I watch him do it and check it, and if he gets frustrated I let him ask for help and remind him that he can, but I don't step in unless he asks. He's learning to ask more, which is exactly what I want to see - I won't be there in twenty years to help proactively, he'll have to ask me or others to help.
- That doesn't mean there aren't consequences of failure, though. If the teacher says she didn't do her work, then she can't do [fun things] until work is done. Don't sit on her, don't force her to do it, but no [fun thing] until the work is caught up. Not to say take everything away - she's 5, that's not really reasonable; but no TV, for example, or no dessert, or whatever one thing will be a reasonable motivator for her and is also somewhat of a natural consequence. We had a simple rule - no iPad until homework is done - and that worked wonders after the initial period of complaining.
- Celebrate her successes. When she does get caught up, celebrate - have an extra dessert or an extra TV show. It's too easy to get into the "I'm a bad kid" mode. Celebrating successes reminds her that she's a kid - good and bad - and that she can overcome. We went out for ice cream when he'd had 3 red days in a row, for example.
- Finally, have structure for her to help bridge the gap. We had a lot of success when we bought some "school supplies" for the house to make it look more like school - we still have the "Quiet Zone" sign that we bought to imitate his classroom up in our living room today. This made him feel really comfortable and I think made it seem like school was just an extension of home, which helped a lot.