Sex = Body fluids. Sometimes all of them at once.
Try not to think of it as a big deal, because it does happen. It used to bother me, having sex during my ‘time of the month’ so, if the urge and opportunity presented itself, I would move the action to the shower. I would have to make sure to avoid the water to not mess up the natural lubrication, but we were both happy and satisfied after the activity and if there was some menstrual fluid, it would just be washed away and no ‘ew’ moment.
These days, just having sex at all, at any time of the month, would be a welcome change.
But seriously, really, when you’re being that intimate with someone there are going to be moments of TMI. Passing gas, muscle cramps, accidently hitting his nose as you change position, .. all sorts of imperfect bodily functions. It’s important to be really comfortable with the person and to make that person comfortable with you. I hope you didn’t freak out too much about the blood in front of her. If you did, you should apologise. She was probably more embarrassed than you.
(Here I am being all ‘old fashioned’ thinking you have a close relationship with this girl. Correct me if I’m wrong.)