Brown stuff on floss after flossing?

I brush and floss twice a day. Lately, I notice that occasionally when I floss between 2 particular teeth, the floss comes out brown. It's always between the same two molars and I have to floss the area a few times before the brown stuff stops coming out. I'm not experiencing any pain or sensitivity that I've noticed. Could it be a cavity? Or just regular floss "gunk"?

[Gellia]

Hi,
It is possible that the tarter that accumulates under the gumline has turned black, which it will do if it is there for a long time.  That is the start of periodontal disease.  You really can't reach it completely with normal brushing and flossing.  Only your hygienist can.

You probably just need a thorough cleaning.  Do get it though.  Leaving that plaque there can start to dissolve the bone holding the tooth in.  

My best to you,
Gellia