Bug bite/ pimple/ idk help

I noticed a little red pimple like bump on my arm yesterday because it hurt when the seat belt touched it. Today it’s been achy, more red, and swollen! The perfect circular redness around it is worrying me and it’s very tender to the touch. Maybe because it’s swollen and red but my veins are almost impossible to ever see and right now they are very visible..... so any suggestions as to what it may be; I have not been bitten by anything that I’m aware of and it’s not very itchy only sometimes!