Can't say Ive ever heard of not wearing underwear causing a UTI, but maybe you could try wearing it more often and see if there is any change. Just my experience, but I dont have those kind of UTI problems and I don't always wear underwear myself, depending on what Im wearing, tight pants, certian dresses, etc. Just sayin.
Might also try some AZO standard, and drink more water, and if it doesnt go away see a Dr. Might also need to get an STD test, if youve been sexually active.
Ive never heard of that either haha umm i do know not drinking enough fluids can cause utis but i kindof see why not wearing underwear would cause a uti because your pants would be irratating down there but idk haha