I had sex with my boyfriend yesterday evening, and i noticed blood in my pant when i wanted to urinate this morning, what could have been the cause?. not my menstruation period?

Bright red blood at any point in your cycle suggests torn tissue with active bleeding. The source is likely linked to sexual activity. If it is minimal & lows down quickly I would now worry about it. If it persists for several days an evaluation would be prudent.

You are a male - so no period. You saw blood on pants before you urinated so -not from urine. The cause could be from any site in the region. So pl check the area

The blood could be from a form of rough sex or a penis that is so large that it scrapes the vaginal tissue causing irritation. Clean area and next time use a water based lubricant.