Brown discharge?

It seems like once a week for the last few weeks, I have found a streak of brown discharge on my pantyliner. Its never ongoing and really doesn't even show up when I wipe, no major cramping or anything. Just one random day, I'll find some and then it stops. I'm 8 weeks now and this has happened 3 times. My last US was 8 days ago. Should I contact my doctor? Like all of us, I'm always on alert and afraid of miscarriage.