Pulled abdominal muscles and brown spotting

I am 18 weeks and 5 days pregnant. About 3 days ago, I pulled a muscle in my upper abdomen, near my ribcage, rolling over in bed. I had shooting stomach pains all day, that went away when I went home and rested. The pains were 100% gone by the next morning. I did not experience any other symptoms other than the pain and my mw agreed that I was experiencing ligament pain and likely pulled a muscle from sudden movement.

Today, I have some brown discharge when I wipe. I have been experiencing brown discharge since 9 weeks of pregnancy, which is worrisome to me, but always has turned out to be nothing. The first two times, was 24 to 48 hours after intercourse, and since then, I have discharge that comes for a couple of days and stops for about a week, sometimes 2. Mostly, I can trace it back to either heavy lifting or intercourse (stopped intercourse on my doctor's recommendation 4 weeks ago.) I have been careful to rest for the last few weeks and am wondering if pulling the muscle in my abdomen could cause the brown bleeding?

Is there anything else that will cause this to happen so often in the second trimester? Should I be worried that something more serious is going on that could potentially harm my baby?