help. advice please.

hello! i need some advice, my boo and i had an accident back in june, he finished in me but i was on birth control. i had my period for june and july and not in waiting to start for august! my period tracker said i was suppose to start yesterday and now it's telling me i am suppose to start tomorrow.... i have been having period cramps and i haven't had anything abnormal happen, what should i do? i know i'm not pregnant because it's been three months since then and i haven't been on my birth control for almost a month. does not being on birth control all of a sudden effect my period and when it comes?