I am praying fervently and dude for the love of God do not, DO NOT let any doctor cut it open and drain it. It pains me to say my mother also discovered a lump under the rib- turned out to be some sort of sack. Doctors did not know what it was so they cut it open and fluid spilled out. Turns out it was full of a rare type of cancer and cutting it open caused it to spread throughout her body. She started chemo but it was too late she was dead in six months. All because of some stupid idiot doctor in my stupid worthless town.
I pray to God whatever it is is nothing, but I did feel the need to warn you. If they discover some sort of fluid filled sack, do not allow them to do a biopsy go to the nearest cancer specialist, and contact me I will ask my sister the name of the cancer my mom had and tell you.
Edit: it would have been easily treatable if the stupid doctor had not cut open the thing and let it spread. Once it was out of whatever was containing it, it spread so fast there was never any hope.