kinda freaking out a little bit!!!

Ok...so my boyfriend has three children from his previous marriage. He had a vasectomy about 2-3 years ago. I'm not entirely sure how long ago. But. We have been having unprotected sex and he doesn't pull out. I started my period yesterday, or at least I think I did... it's extremely light and really dark. Like usually I go through two super plus tampons a day. If I didn't just change my liner throughout the day because it's gross to keep one on all day...I wouldn't even know I'm on my period. Like there's a little on there, but hardly anything. This is not normal and I'm freaking out. Could he still get me pregnant???