It could be the anal glands secretion. The gland secrets this fluid to lub the anus when a bowel movement is made. Sometimes it does need to released by vet or the dog groomer. It's not painful just a little embarassing.LOL The gland is (I beleave) located just below the base of tail inside of the rectum. Inserting your index finger and applying a little pressure to that area is all that is needed. Warning it is gross (smelly,etc) when done! Since your dog is a female she could have a slight vaginal infection. You may want to call your vet to ask his/her advise.