Possible missed period

I have been on birth control for a few months now. My periods are like clock work now except for this cycle. I didn't have sex during this months pack that I can recall, and only had one late pill. My period usually arrives the Tuesday or Wednesday after I finish my pack on Sunday. I have also been having diarrhea really bad these past couple days. Could that affect when I get my period? I'm kind of concerned but not freaking out. I feel like I need to know when I'm going to get it so I can be prepared.