It could a lot of things. If you have changed soaps or detergents recently, then it could be an allergic reaction or contact dermatitis. These can be easily treated. If you are wearing tighter underwear, rushing from completely drying off after a shower or sweating a lot in that area, then it could be jock itch ( fungus related ). This is easily treated as well.
If you had oral sex without protection and if you notice blisters, flu like symptoms then it could be a herpes infection , HSV-1 but let's not go there just yet. STDs can usually be noticed bt unusual discharges, odors, blistering, fever, flu like symptoms, fatigue, tiredness,nausea, vomiting.They can make you feel icky.
Herpes infections are skin to skin contact and if I were you, make an appointment to see your doctor or go to a nearby clinic for evaluation. Don't think about the possibility of Herpes. I'm just letting you know that because Herpes can be extremely mild and can be mistaken for other things.
I hope I didn't scare you but from the way it sounds, it could be an allergic reaction to something.Still check with your doc to make sure and if you have any questions ask away.