Dogs have anal glands near their tails and the glands some times get clogged and need to be expressed. If they become abscesses they can become very painful and rupture. Regardless of where the blood is coming from, your dog needs to see the vet!
All the answers are good here. It can be shes in heat if you have not got her spayed. She could have got a small cut in her bottom or around it from it being dirty. Best to clean if off look to see if its a visible wound on the outside and what colour the blood is. Dark red is more worrying as it comes from deeper inside. Lighter red is closer to the enterance or skin wounds. If there is a smell make a note of that too there are so many things it could be best to get it checked as some are life threatening (iin hours) parvo is one. Is the dog happy in its self eatting drinking urinating and passing feceas iwith if so it might not be a bad thing to monitor her for a little while to see if she is chewing it or anything before getting it checked. If she is not righ though go straight away better to be safe than wish you had gone. Hope it all helps.
Has she recently pooped? Did it look normal? I know they are gross questions, but you can tell a lot about a dog's health by their poop. It could be several things some minor, some not. If she is pooping fine call you vet in the morning. If she is having trouble call your vet now.
I KNOW!!!!! I have a dog!! I know everything!! Here: it's just like a period humans have(girls) its wear there butts bleed.... And what I do with my dog i get them this dog diaper lol!!! And for it doesn't get blood all over. But it's totally normal! Trust me!:)