Worried about prgnancy

Ok guys, I got off the depo shot in march because I didn't like what it was doing to my body. My husband and I have been super careful since then using condoms, etc. We had unprotected sex (we had been drinking, go figure) for the first time since our last baby (2015) the day before this app said I was to start my period. Mind you I think my periods are still slightly irregular from the depo, I've only had 2 periods since but they've both lined up well with the app. Anyway according to the app my period is 3 days late now and I'm beginning to freak out. I already have 3 children and I'm scared I may be pregnant again and I don't believe I can handle a fourth. Advice? Insight? Experience?