It could just be that it is almost time for her period and she released a little blood. Or she could be pregnant. It could just be that her hormones are a little off or any number of things. Maybe the sex was a little too rough for her this time. It is hard to tell.
If she is uncomfortable, take her to a doctor.
Wishing you the best.
Also you could have a UTI sometimes you will pass blood through urine and you think its from the vaginal area but its really coming from a UTI. You should get your urine checked.
Yeast infection or a urinary tract infection. Most likely not an std so don't worry about that but a doctors visit is recommended