I personally would recommend that if you are concerned, call your dr. I had a bad experience from brown bleeding but I was much further along (11w),so it was a bit more concerning then.
It could be implantation bleeding, but I would have thought it would be a little earlier than now.
Sex would more than like be red blood I believe as it can be abrasion of the cervix, so it wouldn't be old blood.

I hope everything is fine