Uti symptoms

Ally

I am 22 (female) and every so often I get UTI symptoms (frequent/urgency but only pass small volume, pelvic discomfort, etc.) but when I take a sample to the doctor it comes back clear. There's only ever been one or two occasions when they've found something (white blood cells I think) I'm going to the doctors to see if I can be referred for tests to find out the cause but does anyone know what this could possibly be?