UTI, pelvic pressure, and black specks in urine?!

This is my second pregnancy, and I'm 28 weeks. I have been having pelvic pressure/pain for 2 weeks now along with urgency and increased frequency to urinate, and little black specks in my urine. They are about the size and color of coffee grounds and it doesn't happen every time I pee. I was diagnosed with at UTI after a urine culture, and been on an antibiotic for a week now and I'm still having symptoms. I go back to the doctor tomorrow, but I was wondering if anyone else has had similar symptoms? I am also constantly thirsty and I am drinking about 2.5 liters of water a day.