Bleeding 12 weeks

I'm currently 12 weeks and have had bleeding for the past 2 weeks. I've gone to the ob and they haven't been able to find a cause. Baby is moving, kicking, and has a hb of 168. I'm becoming so worried as I have had red bleeds for the past 4 nights. They are typically short and turn brown within a 10 mins, but I can't help but stress. I do have some cramping but it is mild and is due to stretching and growth. Has this happened to anyone. This is my 1st and I've been threw so much.