As others have correctly stated, your chameleon has prolapsed.
The color red means it is some type of internal organ that has prolapsed, or inverted on itself and fallen out of the body. A prolapse is a medical emergency and needs to be addressed asap. If the tissue dies or dries out it is very susceptible to infection that can then spread to the rest of the body. If it is a hemipene (male reproductive organ) then it can be amputated if it won't stay in since they only use it for reproduction, not urination or defecation. If it is the rectum or uterine tissue that has prolapsed it needs to be replaced and surgically fixed to prevent future prolapse. If the tissue dies then your chameleon will not survive, as there is no way to fix that.
Keep it clean with warm water and moist with ky jelly to keep the tissue alive and get your chameleon to a vet with reptile experience. Warm sugar water can be applied to try to get the tissue to shrink but if this does not work within an hour do not delay in finding a vet ASAP and keep it moist in the meantime.
Based on the size of your girl's prolapse she is in need of medical attention without a doubt. That's not going to go back in on its own.