cyst on left ovary

I had some red bleeding after intercourse 31/2 weeks ago.  My gyn sent me for a sonogram and found what he called a very small cyst on my left ovary.  He said they come and go and it was nothing to be worried about.  Could the cyst cause the bleeding?  (He thought maybe I had a polyp in my uterus and that could cause the bleeding - no polyp found on sonogram but uterine polyps are hard to see on sonograms) Now I'm 6 weeks pregnant and I had  blood again after intercourse last night, it was very slight and brown and I did not notice it untill the morning. Could the cyst be doing this?  Could the cyst turn cancerous?  Is there  a blood test to screen for ovarian cancer?  If so what is it? Thank you for your help!!  I am 35 with 2 kids with one on the way.  My paternal grandmother had ovarian cancer at 77, and my maternal grandmother had brest cancer at 35.

[much needed mom]

so is his advice to "not worry about the cyst" acceptable or should I check with sonograms continuously? How likely is a cyst which they consider very small to turn cancerous?  Thanks for your info!