I have a 2.5-year-old daughter. She is almost almost 3. She poos her nappy and doesn't tell anyone to the point her bum is red and blistered. Can anyone give me advice or have something similar why she keeps doing it as she use to tell me?
We can usual tell when my two and a half yr old is doing one. He runs to a specific part of the house and grunts in a corner. Often he tells us, but sometimes he's too busy and will even deny he's done it. Just make sure you're not sounding cross when you find out ask her to come and tell someone. Otherwise cream her up and as @threetimes said she'll be wanting the nappies off before long anyway.
My one child blisters easily on her bum. You will have to put some sort of barrier cream or salve on her all the time to help resist that happening. You will also then have to check her often. Sometimes a child will do something & then stop. My child used to do what I asked her to, then she got to about 3 & decided that resistance is where it's at. It's all a process. Some things could be considered regressive, other things are just phases & development.
So my sensitive skin kid who did not tell me when she had pooped was checked constantly for poops, and I kept cream on her all the time. At 2.5yrs, if she is not terribly resistant, now is also a great time to have her try sitting on the potty briefly many times a day. One way I have luck with this is "racing" them to the toilet.
The best news I can give you is that a this age, you are closer than ever to being done with diapers & that will mean the end of raw bottoms. So hang in there. Sooner or later this will be no issue at all & she will tell you when she needs to go.