Thanks for the information
Two possibilities to consider here
First of all an infection of the intestine considering you have noticed reddish colored stool along with a low abdominal pain.
A stool test will be helpful in this case.
As you have been tested for UTI so chances of having urinary infection is low.
I would suggest to get tested for Herpes as well if it was the only test missing in the STD screen.
An infection needs to be ruled out in your case.
So i would suggest a reevaluation by your doctor for these possibilities
Feel free to ask if you have any more question
Please leave a POSITIVE (3 – 5 stars, preferably 5) rating by clicking on the stars above, until you do i am not compensated for my time. You can continue to ask follow-up questions even after the POSITIVE rating