Fully potty trained having “accidents”

Ok so my 3 and a half year old who has been fully potty trained for awhile, has been having tinkling accidents. She use to go to the bathroom whenever she had to go but now she is holding it in until it’s to late and lets a little out. She will immediately go to the bathroom and does fully pee. It’s frustrating though since I have to change her underwear consistently. Is this normal? How do I fix this? I force her to go all the time (which is a huge fight from her but of course she always goes) but I can’t always watch her and make sure she is going to the bathroom since I have a baby as well.