And how can I make it stop 
My DD is (newly) 4 and has been fully potty trained since 2 1/2. Ever since I potty trained her sister about 3 weeks ago she is continually having "accidents". I don't think they are truly accidents or based on a physical problem because she goes to pre-school 3 days a week and does not do it there, or anywhere else. She has had sleepovers at my sisters and been fine. It's happening every day or sometimes twice a day at home.
She will be playing and suddenly announces she needs to go, runs up to the toilet and then will announce she was too late. The accidents always happen in the bathroom, never on the stairs or anywhere else. If we are out she is fine to wait for the toilet, at my sisters she has to wait to be let out of a baby gate and has no problem waiting. No problems at nursery. No bedwetting.
I have tried taking away screen time for accidents but it just upset her and felt cruel. We agreed on a star chart, one for every day dry to get a reward but haven't even been able to get one star! I praise her when she remembers. Her sister isn't having accidents and isnt getting loads of attention.
I just don't know what to do!