Okay I need help

Jenna • Hey my name is Jenna and I`m in love with a soldier! And we are having another baby!
I have a 3 year old girl that was completely potty trained and had been for like 6 months and all of the sudden she is hiding again in the corner to poop when she loved to be a big girl and go on the potty is this normal or what is going on? Why is she doing this?