I've had brown spotting/discharge about once or twice a week since I was 15 weeks. Every time I mention it to my OB he says as long as I keep growing, and I'm not having labor pain/contractions, he's not worrying about it. I've never gotten an explanation of why it's happening. Baby appears to be just fine.
It is likely caused by the fact that there is so much extra blood and tissue and whatnot in and around our vaginal openings, so the slightest irritation can cause slight bleeding. Are you still having sex? Do you take the stairs? Did you sit down too hard? Did you wipe too hard after using the bathroom? Any of these things can cause slight bleeding that isn't any cause for worry.
Re: Brown discharge