What Causes Recurring Blisters On My Penis?

I am 43 years old married man and I am suffering from diabetes for the past six years. I have thyroid and cholesterol also. I am taking insulin injections, metformin tablets etc. once in six months I get blisters on my penis, it swells and become like wound. Skin peels off. it is taking 5 to 6 days to heal after using some antibiotics. I have not consulted any doctor for this trouble. I would like to know why it is happening and how to treat this. please answer. I don't have any bad habits