You semen most likely isn't really becoming more acidic (physiology demands that it be basic) or more caustic (again, the pH will remain very slightly above 7). Your skin could be getting more sensitive over time, though.

Keeping clean is good, but you may have developed a sensitivity to laundry products or soaps you use in the shower.

Be careful about any chemicals you may use that could come in contact with your genital area; try cleaning as thoroughly as you can, for a while, with only warm water. Or see a dermatologist or urologist.