Nervous.

So I'm on birth control to help regulate my cycle and help my reoccurring headaches. The last two months I have started it at the end of the pack like I was supposed to but this month I am two days late. I was involved sexual activities last month the day after my period but we didn't actually have sex. Should I be worried and take a test? Or is my body just trying to get used to be regular? I'm 17 and my mom says being irregular is normal but I'm still nervous.