What Causes Penis Blisters And Rashes During Erection?

Hi Doctor, I am 58, diabetic, broadly under control with medication. I have been diagonised as having mild prostate, grade I. I have undergone medication, and feel that prostate is under control, though am yet to go through a test, if my doctor prescribes (whom I am yet to meet). For the past 2 months or so, I am having a problem. While erection, the foreskin gets red small blisters-like, with slight reddishness at the point of foreskin connect with penis, and one or two red rashes on the penis. The pain due to redness is so much that erection is aborted by self. This pain continues for a few hours; in the meanwhile, if I pass urination, then there is a feeling of hot liquid pouring out. I tested urine today, but it shows no bacteria, everything else remaining normal. Even sugar is found nil. pus cells and epithelial cells are normal. If I do not indulge in sexual act, there is almost no problem, no pain, no burning sensation. Could you please help.