I have a long oval red rash with white film on my butt crack. It looks like diaper rash. It doesn't itch and only burns slightly. Yeast or fungal?

Either or neither. Sounds/looks like an infection, possibly yeast/fungal, can be a side effect of a drug like an antibiotic, or if you are diabetic. You can try OTC clotrimazole cream twice a day, keep the area clean and dry, if no improvement in few days get seen, best wishes.