Skin rash in private areas developed: Tetanus, Measles & flu shot

I’m a 46, male on Paleo with no known allergies.
After getting these shots: Tetanus & Diphtheria Toxoids (Td), Measles, along with the flu shot. Hours later I started to develop a skin rash on my buttocks and crotch area. Why?

During my physical my doctor recommended that I get these shots. The inside of my forearms also have a slight rash, but not as bad as in my private areas.