What can cause redness on the frenulum of the clitoris that causes it to burn when urinating?

A few things really, ether soaps, sprays, accidental scratches or infections, if you wash with soap down there only wash over the top where all the hair grows and in the bum but that's it, and don't use sprays or powders over your private areas, you can use powder between your legs and under your belly but you shouldn't put anything inside, only wash inside with hot water. Sometimes it's possible to accidently scratch yourself when masturbating, i've done it a few times with a sharp nail even if i bite them, but you will want to get tested for yeast and other infections just incase.