Poop Emergency

I dont' know what to do! I need some advice. My daughter will be three next week and after she found out that she will be a big sister she has stopped going in the potty. It sucks!!

She was 85% potty trained (I don't know how I came up with 85%, but it seems like a good number). She was going poo and pee all the time in the potty, she was even sleeping with panties...She did not wet the bed once!!! The only thing, she was a little slow at telling me that she needed to go potty when we were out, but we would get to the bathroom on time most of the time.

Today I tried leaving her with out a Pull-Up to see if she would run to the potty, but she cried and held her pee in until I put a pull-up on. During the process I tried sitting her in the potty, both hers and ours, and she screamed.

I need some advice from mothers that have gone through this. I just don't want to be called in to her high school every time she needs her diaper changed. Help!!!