Could I be pregnant...please read!?

I have started working out in January and I have lost 20 lbs total. Prior to January I had unprotected sex and then in December I got my period a few days late and assumed I was not pregnant because of this. Well the last few months I have gotten my period but it's been really light. I was told this could be because I am living a mre active lifestyle. I took a pregnancy test and it came back negative...here is where my worry is. My stomach is going out way further than it used to. Could this be my abdominal muscles growing? Also I have been feeling werid bubbles in my lower abdomen. Do you think there is a slight chance I could be pregnant??