What Causes Light Period, Bloating And Left Side Of Abdomen Pulsates When Pushed?

I started my period on the day I was supposed to on December 6th but it was only 2-3 days started off light was heavier for a day or so and is light again which is not normal I haven't had sex but I got this was on the 23rd of November close and I'm afraid I might be pregnant I've been killing myself stressing out about it I've had gas I was constipated but its normal now I had been bloated and the left side of my abdomen is hard when I push on it and its pulsating what could be wrong