Mine is a semi rigid lump/bump/thing on the top of my arm just above my wrist. Slight pain to the touch/when pressure is applied but very bearable. I have not hit it or injured it that I have noticed so it is just kind of odd.
Stick a fish knife in it and see if anything drains out. Almost nothing a good fish knife cannot fix.
If nothing drains out or no bot fly larva crawls out you might make a DR appt.