Hi K.!
My son is 3 and went through the same thing. Only we found out that he wasn't in control of his bowel following the constipation. He had a blockage in his colon and the BMs that he was having were just leaking out.
It looked like it could ALMOST be diarrhea so I thought maybe it was a stomach virus because he was also complaining about tummy aches and that he was just deciding on his own not to be poppy trained for a while.
I took him to the doctor and they did an x-ray and sure enough his entire bowel was full of a large blockage. Previous to this he had had only ONE day of constipation.
They recommended we give him 1/2 a teaspoon of milk of magnesia every 4 hours until he passed something large enough to have come out of an adult (that's how the nurse at the pediatrician's office described what I was to look for). We did that for about a week before I was sure his BMs were back to normal (not so loose).
Previous to this incident my son was just finally getting poopy trained, but following this we're having to retrain him. We find that we just have to sit him on the potty at the same times every day (i.e., before breakfast, after breakfast, middle of the morning, before lunch, before nap, after nap, after supper, before bathtime, before bed, etc.) to give him plenty of regular opportunities to have a BM on the potty.
I find, in my son's case, if I don't give him a chance to soil himself, he gets used to having clean underwear.
But my advice is to take you baby to the doctor. This situation may be out of her control.
I hope this helps.
C.