Intense anal itching can be from inflamed haemorriods, fungal problems from sweating a lot and also from thread worms.
Anyone can get threadworms so don't be too alarmed about that.
You can buy from the pharmacist medication to treat and kill off the threadworms. If there are other members in the household, they may need to be treated as well.
Look at your faeces to see if there any white thin threads (hence the name threadworms). Sometimes, they may not be visible.
Always wash your hands after using the toilet and before eating any food.
Threadworm eggs are not visible, but can be picked up from contaminated door handles and other contaminated objects onto to your hands and can get into the body by touching your mouth with your contaminated fingers.
Best wishes
So sorry - how uncomfortable! Could it be caused by the warm weather? The humidity is causing all kinds of fungi - try some gynelotrimin to see if it helps. And use baby powder to keep your back area dry.
If this doesn't help, you'll have to see a doctor to determine what kind of fungus it is. Or if it's a heat rash or a parasite.
Again, I am so sorry that you are having this problem.
Wishing you the best.