Unfortunately, there's not much to do except see the doctor. Has your doctor done betas to be sure they are doubling properly?
You may have just been early at your ultrasound (could have ovulated later than you thought or implanted late and not be as far along which could cause you to see less in the ultrasound than you were expecting). Is your doctor doing another u/s around 7 weeks to check if there's been growth and progress (a heartbeat, a fetal pole, etc)?
I do know progesterone suppositories can cause some spotting too.
I did have pink/brown spotting mostly when I wiped, occasionally a few drops on a pantyliner throughout the first trimester (until 13 weeks) and am at 39 weeks now. I wasn't on progesterone, doc just said some women have a sensitive cervix in early pregnancy. In my case, the spotting never did get heavier, would go away after a day or so (once I was beyond 5 weeks) and then return a few days later.
Did you call the doctor? It sounds like you should call and go in to get checked. Some cramping in early pregnancy (like period cramps) can be completely normal, but it sounds like with your history, you should get checked out.