Do you by chance have PCOS? My girlfriend has it and it causes her these problems, along with the cysts. She's never had a bleeding one, but I can't imagine it'd be too serious because it is so small and it'd probably go into your urine? (I'm a little hazy on my female anatomy, forgive me, just trying to help.)
If the doctors aren't concerned, I wouldn't be. Just keep an eye out for any abnormal bleeding or pain.