Potty traning

I have a almost 4 year old and a 5 month old. My older daughter stop going potty when my younger daughter was born. She will hold going #2 for days. My pedestrian told me to give her warm baths. After awhile she started going again. She was pretty much potty trained but just a week ago she told me she was not going potty any more and just stop. She also has continued to hold herself from going #2. I unfortunately end up in a power struggle with her but I'm not sure how I should handle it. My husband and I really appreciate your help. Thank you!