I Found A Hard Lump On The Posterior Part Of

I found a hard lump on the posterior part of my vagina back by my cervix. It is not on my cervix but not too far from it on the posterior side. I can feel it through the vaginal wall, but it's not painful. Just hard and rounded and about the size of a quarter, and therefore scaring me!! It was not there 12 months ago. If I had not been feeling around in there for my IUD string I never would have felt it. What is it??! I called my gym and left a message but it is the weekend. Help! Worried!