Pregnant ??

So I recently go on birth control about a month ago, and the doctor said it would he affective within a week. So me and my boyfriend waited like two weeks and he's cum inside me about 5 times now.

I am really worried now though because I am late on my period one day and I'm freaking out. I am having stomach issues like I normally do when I am about to start my period but I am not getting cramps like usual. The doctor also said I might spot or skip period and over the past month I've spotted here and there. Could that have been my period and I just didn't know? What would y'all do? And when should I take a test if y'all think I should. I'm really worried because I'm 18. I just graduated and I can't have a baby right now.

UPDATE

I am now three days late and I'm seriously getting worried now. What should I do?