My Fiance Has Been Displaying Sudden Outbursts Of Anger Lately

My fiance has been displaying sudden outbursts of anger lately in response to minor incidents. Some have been too extreme, leaving me worried that he has a mental disorder of some kind. When I ask him why, he just apologizes and says that he doesn't feel well. One example, we pulled up to get gas, he couldn't run the gas pump. I was walking in to prepay for the gas. He slams the nozzle on the ground yelling that he just won't get gas here.. he would rather pay a higher amount than to be ignored. I set the gas pump up for him and the only thing he had to do was squeeze the nozzle. I was extremely embarrassed as everyone in the parking lot was looking at us. Thoughts?