What Causes Vaginal Spotting After Urinating?

Hi, I am a 46 yr old female, 5' 2" 125lbs, and I had a UTI a couple of weeks ago, Ive since had blood work done due to migraines, dizziness etc... Blood tests were all find except my Vitamin D was low so Dr. prescribed 50,000 U once a week for 56 wks. I have been spotting and wiping after urinating brown blood. Im not on the pill, and not pregnant. Is this serious??