recurring UTI

I'm currently 24 weeks and every time I have gone to the doctors (every 4 weeks) since I conceived I've been told I have a UTI and am given a prescription for antibiotics to take care of it. I have not once had any symptoms of a UTI, but always take my medicine as prescribed. I don't understand how this can keep happening... I haven't changed anything in my personal hygiene since becoming pregnant (other than using a different shampoo), and have even tried taking extra precautions to avoid this but it keeps happening. I feel so dirty! I've stopped filling the prescriptions because it's too much money and just doesn't seem worth it when I don't have a single symptom. Could it be that I don't actually have an infection and that something is just tipping off the tests?