I had unprotected sex with my boyfriend on the 26th of January. My cycles are usually regular (between 28-30 days), so I expected my period to arrive some time around January 30th. It didn't. I got some very light spotting - the kind that usually warns you that you're about to start - around February 14th, and that lasted three to five days. I assumed that this was my period, though I usually have quite a heavy flow. I have not had a period since then, and I haven't had any noticeable signs of ovulation either. I've taken several pregnancy tests - one right after my missed period, two in the week following my missed period, and a couple tests in March. All have come up negative. Could I be pregnant and just not showing the signs, or could there be a problem with my system?

Addition: I forgot to add that I had surgery in mid-March to remove my tonsils - could that have anything to do with all this?