I've had multiple tests (urine sample) for urinary tract infections and nothing ever comes up, even though I suffer all the symptoms. What could it be?

Interstitialcystitis. You probably suffer from interstitial cystitis. Consult a urologist but also see a holistic practitioner as this condition is often related to food sensitivities, intestinal yeast overgrowth ; chronic infections like lyme disease ; can respond to herbs, homeopathy ; other natural therapies. See https://www.Ic-network.Com/conditions/interstitial-cystitis/ ; http://curezone.Com/blogs/f.Asp?F=1421.