Bleeding mid cycle

I am having some bleeding on day 14 of my cycle. Like actual red blood, not brown spotting. It was enough to go through my underwear and reach my pants that it was very visible. There is no more blood now, so it’s not a continuous flow.

I’ve never had this before. I am 35 years old and have 3 kids and never had any menstrual problems or irregularities before.

I am not on birth control but I have been sexually active (my husband had a vasectomy 2 years ago so unless it failed, it’s unlikely I’m pregnant).

What could be the cause of this??