Spotting for 3 weeks now.

Hi ladies I should be 8 weeks pregnant today. I go back Monday for an ultrasound. I have been spotting for 3 weeks straight now from pink to brown and red. Not heavy at all but definitely there. And cramps were very suttle and light. I am so nervous that I had a miscarriage. But if I were to have a missed miscarriage or miscarriage wouldn't I bleed very heavily? I also don't have any symptoms. I did have very gas and my breasts were killing me up until 2 weeks ago. My pregnancy tests are still coming back very positive and clearblue is still at 3+. So I'm very confused. My last scan showed a gestational sac and yolk 2 weeks ago.