hello and thank you for using this site to help you. My name is ***** ***** I will be happy to assist you today. I see that you are off line so I will answer you the best that I can based on your question.
There are a couple of things that can cause this, one would be an allergy, either food or contact, fleas, or a bacterial/fungal/ yeast skin infection. In order to determine if it is bacterial, fungal or yeast you would need to get a skin scraping done by your Vet. This is done right in the office and you will have an answer before you leave. Each requires different treatment, bacterial may require antibiotic meds, whereas the other two require bathing in specific shampoos. If there is any licking or scratching going on I would suspect bacterial.
Food allergies and contact allergies will also show rashes on the abdomen and inner thigh areas. Usually with food it is something the dog has been eating all along, not a new additive. Most dogs are allergic to beef, rice, and chicken, so there are dog foods made specifically for dogs with skin allergies such as Duck and Sweet potato.
I am going to direct you to a link on food allergies and if you go to picture number two on the bottom of the page you will see what the rash looks like.
IF there is redness or swelling around the anus then her anal glands may be full or irritated. Your vet can express them for you. Often times these are expressed through a bowel movement but if the stool is too soft the sacs may not break open. Adding a little pure canned pumpkin to the daily diet can help with this. Sometimes dogs will scoot along the ground if this area is irritated.