What can be wrong if theres blood in my urine?

Many things. Blood in the urine could be from many things - infection, polyps, tumors, even vigorous exercise. This is such a common finding, however, work-ups and treatments are well-documented. Sometimes we never find out why...But assuming anything bad is ruled out, that can be ok too.
UTI. The most common thing would be a uti. Other causes for hematuria: kidney stone, idiopathic (don't know) or bladder cancer.