You know what, fellow mommies? Not just you N. -- really, not just you. 3 is very young, 3 is a big girl, but really, still a bit of a baby in there. I'm going to veer into my own experience for a bit, and I hope you don't mind: I personally can remember going through a time when I was 4 and we'd moved and were living in my aunt's house with some resentful cousins and I needed a diaper at night for a little while. The night my mom said, "Ok honey, it's time for you to go back to your panties" (or whatever she said), I had the conscious thought, however a 4 year old thinks it, "Oh, this is it -- no more relaxing." I'm 52 and I clearly remember that feeling -- I felt sort of hung out to dry.
So imagine a 3 year old and all that developing they still have to do. I really, really believe they just aren't ready for full potty training, no matter how well it's gone up till the accidents in question. Not just because of my own experience. I've watched so many of my mom friends struggle and their kids just resist and put up a fight. A 3 year old has very little impulse control -- some 5 year olds still have very little impulse control. I know it's a hard thing to have 2 kids to diaper and 2 tushies to wipe, but I really, really hope you don't get mad at her when this happens. She's probably just so comfortable when she wakes up from her nap and it just comes out of her. Then she gets scared and tries to hide it. Imagine how she feels.
Please don't make this a discipline issue -- for her or for you (by that I mean don't be hard on yourself & I very much hope my letter here isn't coming off like a lecture). I think it's very normal for her age. My daughter's pediatrician said if I just relax my daughter will potty train herself. Well, I did make a few suggestions to her, but she wasn't ready until 4 -- that may sound like a crazy long time to put up with it, but I did it. She was of course potty trained by the time she went to Kindergarten, with a couple of pee accidents after that, but it was very easy, I must say.
Try to see that your 3 year old sees the 7 month old being babied. I don't know how much difference she is able to see between herself and her little sister. She may feel a bit pushed to hurry up and grow up. I hope you'll just give her some more time in the pullups, and lots of love & compassion until she's ready to get it perfect.
I wish you & your family all the best,
Colleen