Slight brown spotting 16 weeks 3 days

I'm super anxious. When i was around 9 weeks i had some blood and went to the dr, they couldn't find where it came from but baby was fine. Then again at 11 weeks I didn't have blood but more so brown spotting for about 5 days I went to the dr and he looked and everything was fine, he says he isn't concerned unless I have bright red blood and it's filling up a pad, he said it could be from a broken blood vessel or really from anything but that it was old blood. So now at 16 weeks and 3 days I woke up this morning and I had a very light amount of brown spotting discharge. I've been checking through out the day when I wipe and it's either not their at all or an extremely small trace of it. I don't have cramps or any discomfort. It's such a small amount but I'm worried because I'm in my 2nd trimester and i dont feel like I should be having this. Has anyone else had this and everything was fine? We just heard babies heartbeat a week ago. And i know each time this has happened it has not been from sex because there has been a weeks worth of time if not more from having sex to spotting