Hello,
It can be due to sweat dermatitis, moisture(due to sweating or frequent washing), the abrasion caused by clothing, the pressure of sitting, chemical irritants like laundry soaps, colognes and scented toilet paper, certain food irritants, infections like STD’s and pinworms, anal abrasions and fissures and skin problems such as psoriasis, seborrhea and eczema.
Apply cool compresses on the area and take bath with cool(not very cold) water. Do not scrub your skin or shave your skin and use a soft towel and gently pat your skin dry.Also use a sunburn Over-The-Counter remedy containing aloe vera and a light moisturizer or a dusting powder to relieve chafing.You can apply silver sulfadiazine (1% cream) on the skin except face and see if it helps.If the symptoms still persist then you should consult a dermatologist and discuss the option of dermabrasion or laser resurfacing.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
personally I get swelling near sunburns, I suggest you take it easy, take plenty of fluids (sounds counter intuitive but it may help) and see your dermatologist or primary dr. also treat the sunburn with plenty of aloe