How soon can I expect to see my next period? starting to suspect i'm pregnant?

My last period started on May 22. I've been on birth control for at least 3 months now and i take it like i'm suppose to. i'm on birth control but me and my husband haven't been using condoms either. i'm starting to suspect if i'm pregnant now because i've been nauseated a little, having slight cramps from time to time, i got light headed a few times standing up this morning, my breasts are sore to the touch(only when you apply pressure), and i've been irritable as well. at first i thought these were all just signs of me about to start my period soon, but how soon can i really expect it to come? my period usually comes every 24-28 days. i'm trying to keep a close watch on my body so i won't miss anything, but i'm starting to get the idea that maybe i need to be looking at buying a pregnancy test since me and him haven't been using condoms at all. and i've let him go inside of me a few times. expect to buy a pregnancy test?