I have heard of women getting pregnant with the most impossible odds - so it's not as unlikely as you may think. What I always think of in such instances though, is that this is a life that was truly meant to be and entrusted to you especially... pretty cool stuff.
Please post again and let us know what happens. Either way, you should see your OB/GYN - there are other things that could be going on (like an infection). And you should be seen by a professional.