I'm on birth control but I'll admit that I have not been taking it like I should be. I do have unprotected sex a lot. Anyway, me and my boyfriend had sex and he came inside of me, and the next day I have a brown light period for about 3 days, Could that mean I'm pregnant or else could it mean.

If you have unprotected sex, then the answer is "yes" you could be pregnant. On the day after, was your period due? It could be spotting before your period. Bleeding right after sex could also be an STD.
Hope this helps a little.

We've been together for awhile and both been tested so I don't believe it is a STD. but my period was about a week and a half later than usual. And I usually have really bad cramps and didn't have any this time.

How long have you been on birth control? Because does have a tendency to make periods lighter and less bothersome. But it also often regulates your period more too. I'd play it safe and talk with your doctor just to make sure it's nothing. But at this point, I wouldn't stress.