What Causes An Over Active Bladder?

Hi. Last Saturday woke up with a sense of urinary urgency. Having had UTI s when I was in college, I figured it was the beginning of one even though I have not had a UTI for approx. 10 years. I bought an over the counter uti test which showed strong positive for leukocytes. I then started on keflex because it was what I had in the house and I read that it can be used for UTI s. My symptoms did not lessen so on Mon. I contacted my general physician who ordered a urine culture. This came back negative for urinary tract infection according to him. I took another over the counter test and it also showed negative. I started on Macrobid on Wed. and am finishing the 7 day dose as prescribed. My problem is that I still have the urgency. I never did experience the pain or burning normally associated with a UTI and have no other kidney or bladdar problems that I am aware of..I am very proactive about my health and see my general physician every year with bloodwork. I also take synthroid due to hypothyroidsim per my endocrinologist. I had a double mastectomy Sept 22 due to DCIS with intrusion but am currently cancer free. I am not on any other medications than the synthroid, claritin and as I mentioned the macro bid. I have had these urgency symptoms for over 7 days now which is annoying and uncomfortable. I have an appt with a urologist tomorrow but I am scared it s something awful that is going to require physically intrusive testing. I am only 48 and have never had any over active bladder symptoms or weakness of bladder etc.