I have had a cut and put a plaster over it for 4 days when i took it of i had bad red bumps that itchy how to get rid of them they on my knee?

It sounds like: your skin had an allergic reaction to something in the plaster (bandaid?) or something you put on the cut (antibiotic ointment?). If that's the case, they will go away on their own in a few days. OTC steroid cream and oral antihistamine can help in the meantime. I suggest not treating a wound this way in the future. Best wishes. Dr. Anne