What Causes A Red Rash On The Penis Head?

I am a 41 year old uncircumcised man in a stable relationship. I have celiac disease and occasionally dh outbreaks. Over the past year I have noticed at different times there is a pink/red sort of rash on my penis head. It is sometimes tender. No STD and once the dr said yeast and the meds worked but this is not exactly like the yeast, just a rash. Not itchy. My fiance says the head looks normal to her and that what I am seeing is normal. Is it normal to have anything like this?