If I were you I would go to a doctor
What Causes A Dark Purple Bug Bite?
Don't know, but if it's persisted for 5 days and is growing, I'd have your doctor check it out.
I can't guess what creature could have infected your bitten part of body as I am not aware of your environment elements(insects etc.). I think you should not waste time any more, just go straight to some good doctor and get yourself examined. Don't ignore your problem.