What Causes Frequent Urination And Blood In Urine?

ok.. so a few weeks ago i had blood clots and tissue in my urine and it really hurt to pee and all my symptoms matched to a urinary tract infection (i looked up the symptoms on the internet) so i went to the doctors and my doctor didn't listen to what i told her she said that due to my age (18) it wouldn't be a urinary tract infection and would probably just be my period - which i know was nothing to do with my period! so she put me on anti-biotics and those worked and cleared it up. but 2 days ago i got the exact symptoms and i haven't gone back to the doctors yet because it doesn't hurt any more there's just a small amount of blood in my urine still and i suddenly get urges to urinate really badly. could it clear up all by itself? or do i have to go back to the doctors?