Uti???

So a few weeks ago i was experiencing very very severe pain in my lower stomach and lower back and it hurt when i went to pee. i felt like i had to pee constantly and did very often. i thought it was kidney stones or a uti. i went to the doctor and they tested for everything possible, even stds. and everything was negative. after a little bit it all went away. but just a few days ago the constant urination and pain when peeing started again. anyone have an idea?