Mysterious lumps on genitals, STI or something less severe?

In the past few days I've had a lump develop under the skin of my gential area, quite near to my pantyline. The lump is quite flat and does not hurt or itch (unless I apply pressure to it), and has some tiny white markings on it. I have experienced something similar in the past, however they were more like spots/blisters and popped, they have left no scarring. I am booked in to have this lump checked out, but I would appreciate any help from anyone.