Mine turned out to be candida and parasites. Intense itch and biting/stinging sensation inside and outside rectum.
The first thing you should do is a colon cleanse. DE and P&B shakes are a good start. The more you poop, the better. Aim for a minimum of twice a day, but preferably more.
Make sure you are following the strict Elimination Diet the dr probably told you about. No nightshade veggies, any type of tomato products, peanuts, tree nuts, all caffeine, decaf coffee, alcohol, sugar, and limit your dairy consumption. Do this for about 6 weeks. Once you are on the mend you can go back to your regular diet.
Scrub your house cleaner than it's ever been. Change your bed sheets every day or as often as possible. Shower when you wake and before going to bed. Use hypoallergenic soap without fragrance. After you dry off, apply some coconut oil to the area and put a couple of small pieces of a clove of garlic in your rectum. Let the cut garlic sit out on the counter for 5 minutes before inserting. This allows some of the enzymes to activate. Use the coconut oil every time you shower and after using the bathroom. You can add a drop or 2 of tea tree oil to it, but quit if it causes irritation. Where fresh pajamas every night. Do not sleep naked.
Immediately start taking any antifungal, antiparasitic foods, herbs and supplements you can get your hands on. Garlic is wonderful. Add some probiotics because a lot of antifungals also have Antibiotic properties that will disrupt your gut flora.
After a week of colon cleansing start taking a daily dose of pyrantel for 3-5 days. Try to do this around the full or new moon. Starting 2 days before moon and continuing for 3 days after moon gets good results. Repeat the 3-5 days of pyrantel once a month for at least 3 months or until you no longer itch. Depending on the severity of your case you will need to adjust how many months you need to continue the pyrantel. Use the coconut oil/garlic inserts as often and as long as needed.
There are many other things you can add to this regimen, but this is a good starting place. Do not mention parasites to your doctor. They will think you are crazy. A simple skin scrape of the area can determine how much yeast is present. I was covered in it, but the real culprit was parasites that I found out about myself with the use of DE.
Avoid all steroids. This includes hydrocortisone anti-itch creams. Do not use any anti-itch creams. Do not use Vick's vapor rub. They thin the skin and will make it worse in the long run. Coconut oil is fantastic!
Your situation may not be as bad as mine and you could be cured a lot sooner, but I was fumbling through this on my own and it took me 2 years to get a handle on it. I still have parasites, but thankfully that excruciating itch and biting sensations are gone.