Periods and irregularity

I generally get my period between the 21st and the 26th of the month. Earlier this month, maybe two or three weeks ago, I had protected sex. For some reason however, I've been incredibly anxious. These were my first sexual experiences, and I suppose I'm very nervous that despite the condom usage, I could get pregnant. I'm having my PMS symptoms - cramping, my lower back hurts, my legs and knees are sore and aching - but I've also heard that pregnancy symptoms are very similar. I won't lie, I'm afraid to get pregnant so unexpectedly, and maybe I'm making a whole lot out of nothing. I guess I'm just looking for some reassurance. Is this just PMS, or could pregnancy symptoms occur so soon? Is everything alright here?