What Does Brown And Red Stain After Intercourse Indicate?

I had sex with my girlfriend last week and after I put my clothes back on Things were normal. I went to urinate and I found like a brown/red stain on the front of my underwear. We thought it was her blood so I just let it go. Today Everything seemed fine and I went to use the restroom and my underwear has the brown/red stain again. What could be the possibility? Can it be a UTD or a STD? Something else? I am 23 years old and I am a white male