Hi there - I'm Dr. Sara. It appears Rebecca isn't online right now and I don't want you to have to wait too long for an answer, so I thought I'd help out. I'm a licensed veterinarian who works exclusively with dogs and cats.
If Sara isn't spayed yet, it could be that she is coming into heat. During a heat cycle their vulva becomes very swollen and many dogs will lick quite a lot or scoot their bottoms on the floor.
If she's spayed, she could still have a little irritation around the folds of her vagina. The saliva from her licking will also stain the fur a brown or red color. It's reasonable to take a baby wipe or warm wash cloth to gently wipe the area clean. It's a moist dark place, so it is prone to becoming inflamed and sore if it stays too moist or has too much hair.
Another reason dogs will lick their bottoms is anal gland discomfort. They have two glands that produce a stinky liquid that can sometimes get backed up and need help to empty. Some dogs will lick under their tails to empty their anal glands (or try to).
If she's really licking quite a lot and keeping the area clean and dry isn't helping, I'd contact your vet. Sometimes they will lick back there because of allergies or infections that the vet can treat fairly easily.
Please let me know if I can answer any other questions for you :)