My boyfriend tried putting his penis in from the back, the tip went in the vagina instead. after 10 minutes, in the bathroom i realized 2 drops of blood. but before this incident the tip went in before and nothing happened. what could explain this?

If intercourse were attempted using rectal entry, injury to the lining of the rectum with eventual vaginal insertion can cause slight bleeding. If only insertion into the vagina is attempted, and this is usual practice, bleeding will not be caused in general if rectal insertion is not attempted.

It might have slightly injured your vagina. Sometimes that can happen

If a lubricant wasn't used than irritation can occur from this angel and cause some bleeding. Whether anal or vaginal penetration from the rear, use a water soluble liquid or jelly to reduce friction. Maybe your period was starting or the angle caused his penis to be irritated, thus the blood.