First of all, is it deep in the muscle, or can you feel a ball under the skin? If it's in the deep muscle (1 inch or deeper, maybe), you might also have a hematoma (broken vessel and bruise deep). That feels like an abscess, and it hurts like F*** and lasts a couple weeks or more. But I don't think I've heard of anyone getting a fever from those. I've had two of them. If it is an abscess, and it's NOT getting any smaller, or IS getting bigger, I'd go to the doctor. I doubt the doctor will believe the thumb tack on the chair story; especially if you're all buff, so tell the (partial) truth. Maybe say your GF dared that you couldn't take the pain of a needle, and you accepted the dare, or something, and got B12.
A Penrose drain may have to go in, which stays in there for awhile. It won't hurt because they'll numb it all up first, and then use a scalpel to dig a channel down to the abscess, suck out as much as possible, and then close it up with the drain in place. Then when it's healed up, they can just pull the drain out and put a band aid on it.
Charles