How To Cure A Lump Inside The Inner Arm?

I have a lump inside my inner arm, where my elbow crease is. It is approx 30-40mm, perfectly round, and somewhat moveable. The lump itself is hard, but it moves a bit inside my arm. It is not on a joint/bone/etc, sort of floating around in the skin/fatty area on the inner part. It is painful to the touch, feels similar to a bad bruise. I haver had any trauma to the area that I can think of (though I am active with my 2 and 4 year old so it s possible). It s appeared in the past 48 hours and is becoming slightly more painful (slight numbness on that area of my arm). Help! Any ideas? No health issues, female 32 years old.