Please help!

My boyfriend and I had unprotected sex the 13th of Aug. ( He pulled out ), it's now the 26th of Aug and eve is saying my period is a day late. About 3 or 2 days ago I had brown discharge which I've had before with my periods in the past. I have no symptoms of pregnancy other than tender breasts and bloating but my boobs feel like they do before my period anyway and the bloating has stopped. I've only had the app for two cycles and this would be my third. I think my periods are pretty regular but I'm not sure. I'm also not sure when i can buy a test. What should I do? I've been pretty anxious and stressed the whole month of august and I was sick for most of the day Aug 12. Do you think these factors could affect my period? Please anyone's input or past experience would be greatly appreciated...