Hello,
From the symptoms it can be a urinary tract infection or fungal infection of the area. To diagnose a UTI please get a complete urine examination and urine culture done. Wash the areas several times with fresh water. Do not use any cosmetic products at the sites. You can apply some calamine lotion at the rash as it will help in soothing the skin. Also wear cotton undergarments. Application of mild corticosteroid cream and oral antiinflammatories like ibuprofen/acetaminophen are also helpful but they should be used only once the diagnosis is confirmed from a doctor.
I hope it helps. Take care and regards.
it sounds like a UTI, sex can increase the likelyhood of getting them, so i advise going to see your doctor who can then test your urine for blood, protein and white blood cells and if its positive then you need antibiotics to treat the infection. if it gets itchier and/or you get a white thick discharge, then it may be thrush and for that i recommend using natural bio yogurt, no sugar and no flavour. its much better than using the over the counter creams etc.
Hope that helps, anymore questions then mail me and i can try answer them :)