More Potty Training Headaches

I'm in need of some potty-training advice. Not that I've thoroughly perused books on potty-training but the ones that I've browsed mostly addressed getting started on potty-training and not my specific issue. Scarlett has been "potty-trained" for about 5 months now. She's gotten to the point where she rarely has pee-pee accidents but she seems to have regressed dramatically with pooping in the potty. We've always had issues with this because she is regularly constipated (which isn't helped by her picky eating) but at least she used to use the potty when she knew that she needed to go. This past month, she seems to have gotten worse and simply doesn't use the potty EVERYTIME she needs to poop. It's not a matter of trying to make it to the potty and not quite making it - she just does it now in her pants and doesn't bring it up at all. When questioned, she denies it completely. Sometimes, I'm calm about it (like I'm supposed to be) but sometimes, I'm just so frustrated that I get furious. It's not as if we forced her to be potty-trained when she wasn't ready. She had shown signs of readiness and we went through it successfully (she had accidents but I learned that it's normal for that to happen). Now, she's regressed and there hasn't been any changes in her life YET! I was looking forward to her not regressing in her potty-training when her baby brother comes because she started potty-training well in advance but now, I'm getting worried. Her baby brother hasn't arrived yet and she's already regressing. Do I need anger-management class if I can't seem to deal with her consistent potty accidents?