A little confused about my period & concerned for possible pregnancy

So my boyfriend & I have been having unprotected sex most days from the 4th-10th. I'm not on birth control, but he always pulled out in plenty of time. One day we tried anal & he finished in me. I am worried that some of his cum got on my underwear & into me. I got my period mid afternoon on Wednesday (12th), but it was super super light, & there was also some brown discharge. I didn't really need a tampon, a panty liner was enough. Thursday, it was a little heavier, but still very light. Yesterday was mostly the same as Thursday, but last night, when I took my tampon out that is had in for a couple of hours, there wasn't much on it, like there were some areas that had no blood on it. My periods usually last 5-6 days, & are usually very heavy for the first 3/4 days. Today my period has been very light, there's barely anything on the tampons. It's also been more brown than red. I feel like what's coming out on the tampon now is just the stuff that's like on the walls, if that makes sense? I know that periods can change, but this is very odd to go from consistently really heavy, to very very light.

I am pretty sure I am ok, but I'm still unsure. Is there any possibility that I am pregnant? Because him and I are both a little concerned rn.