Do you think I could be pregnant?

Ok - I think I probably being a bit paranoid, but here goes anyway. Like the smart girl that I am, I ended up having a one night stand with a friend on my birthday in the beginning of May, the 5th to be exact. We did use a condom and I am on the pill - if I am remembering correctly I had just finished my period as well. However, I have begun to worry that I may be pregnant within the last few weeks. The ONLY reason I feel I might be is because I look and feel quite bloated and my lower abdomen appears a bit different than usual when I am simply bloated.  I should probably add that I bloat quite frequently and have considered going to the doctor to get that issue checked out pre pregnancy worries.

Anyway, aside from my breast getting slightly larger, which I believe is likely related to the fact that I started taking birth control for the second time in my life in February, I have had no signs or symptoms of pregnancy. I feel completely normal. I pee frequently, but I have always been a frequent urinator. I have been getting head aches, but at the same time (late afternoon) and under the same circumstances (usually when there is low pressure) as always. I have had no nausea or odd food cravings. On top of all of this I have not missed a period since the event. I have gotten my period on the third day of placebo pills as I have since I started taking the pill in February. Before I was on the pill I also always had a very regular period - 29 day cycle. I like to think that I am very in-tune with my body and I would generally just go buy a test or head to the doctor, but I am a bit strapped for cash these days. I though I'd get the opinion of who ever is generous enough to respond kindly before taking such measures. 

I guess after all of that what I am asking is this:

After 16 weeks and having had regular periods, do you think there is any chance I could be pregnant and not know?