I'm A 25 Year Old Female. I Noticed A Large

I'm a 25 year old female. I noticed a large lump/hard mass under the skin about 2 weeks ago which goes from my groin/next to my vagina to my lower left buttock. It is painful when pressed or when I am sitting/laying down or standing for a long time. It cannot be seen externally, but when felt it is like an egg in size and is very firm. What could this be?