My children both had a regressive phase in their potty training. The doctor told me that it was normal. Some children are doing a really good job for a while and then they just decide they don't want to try anymore.
The worst thing that you can do is to put the child back into diapers because it will make the second round of potty training even worse. Pull ups are still fine since you have been doing that, but as hard as it may sound, you will have to figure out a system to continue potty training.
Children need consistency, and they need their parents to show them that. If you have to, take the child to the bathroom every half hour - which is what I did. If you don't have the schedule for it, just make sure that daycare is taking her every half hour.
My son used to have a holding it in phase too, but that was because he went to daycare and was too shy to go when he was there.
Is she still going to the bathroom though, even if it is in her pants? If she isn't peeing like normal, she could be getting dehydrated.
If she still goes, try a sticker chart. You don't have to buy anything, you could just tape a piece of paper to the door and write the days of the week on it with a time schedule. Next to the times of the day she goes, give her a sticker or a big check mark with a colored Sharpie and congratulate her!
Kids love attention! Just remember the more good attention she gets for going potty, the more likely she will be to continue on the right path.