What Causes Reddish Bumps With Blisters On The Arm?

I have to big bumps on my arm about the size of a nickle that are red and raised off the skin but each bump has a cluster of smaller little pus filled bumps my arm is very sore where the bumps are even a shirt rubbing on it is irrating what could this be from I woke up with them but the night before my arm was itchy in that spot