Possibly this started as hemorrhoids, which causes wetness in the area, which sets the stage for yeast, and it is moving forward. The scratching and moisture seems to have started a UTI now.
Try a cream like Lotrimin to the affected areas, moving from front to back. Try cranberry pills for the UTI. Wash towels and underwear in hot water with Tide and baking soda.
Get the hemorrhoids treated by a proctologist or general surgeon.
it seems like an STD or maybe an allergic reaction to the detergent you are using to wash your underwear. Try using another detergent if that doesn't work then try to go to your obgyn to get ruled out for STD, if you are clear from them, she can give you some medication (lotion) for the itching. I wouldn't suggest you using an over the counter itch medication being that the vagina and the anus are very sensitive areas. I once had itching in my vagina, I was scared as well and after that I discovered it was because I was using Clorox to wash my underware. Once I eliminated the Clorox, everything went back to normal. Good Luck!!
also when i went to pee it burned!!