What Causes Lumpy Vaginal Discharge?

Hi, i'm a little embarrassed asking this but i have no idea what this is and i am scared to DEATH. 4 months ago i had a baby and since then i have not had a period (one time of spotting) and about 3 months ago i was put on the depo shot. well tonight when i was making dinner i felt something drop out of my vagina, when i checked my underwear i had a big (probably bigger then a quarter and pretty thick) lump. it was hard but yet soft at the same time. what could this be ? there was no pain whatsoever BUT i would get some pain during sex, and now that it came out i am having some brown bleeding.