Did a pregnancy test, was negative. Feel faint&shaky, I'm constipated, have nausea, &urge to urinate. Dr did urine culture, nothing came back. Whats wrong?

Diet. You are severely vitamin deficient and this is a reflection of diet, particularly sugar and sweeteners.
Complex. 20 yo nulliparous with shakiness, lightheadedness, nauseousness, constipated, urgency & negative pregnancy & culture urine (presumably no sugar of diabetes). Absent fever & assuming symptoms chronic, differential diagnosis includes thyroid disorder & Hypermobility Syndrome (often associated with mitral valve prolapse, POTS, IBS-C, & Interstitial Cystitis). See Osteopath. ER if fever, faint, vomit.