What Causes Missed Periods While UPT Is Negative?

Hi i have never missed my period before and i did this month i am 7 days late and i have no pregnancy symptoms besides frequent urination, and tender breasts. i usually get this anyway just before my period. i tested a day after i missed and it was negative, then texted 5 days after i missed and there was a very faint 2nd line which then disappeared when the stick was dry. I am confused. By the way i was diagnosed with Uti about 4 months ago so maybe that is the reason behind the frequent urination, but its not cloudy or painful. I had been trying to get pregnant since last year May but we had stopped focusing on it but i still wasnt on the pill or any form of contraception. my last period was 29 december 2010 and we had sex on the 2nd of January so this would be a miracle pregnancy, exciting though! Please help!