A full 'geriatric' blood panel should be done to assess body functions - especially for liver and kidney functions. A stool sample should be examined for parasites (a 'direct' smear as well as a 'float' should be done)
Blood from the anal area may indicate anal gland problems or possibly internal polyps - a digital rectal exam along with anal gland expression should be performed. Also, a rectal cytology may be warranted.
An easy-on-the-system short-term diet would be boiled white rice with boiled, skinless chicken. This can be fed, then gradually add in her regular diet over a few days to switch her back.
If she's overweight, reducing her weight will be very beneficial.
Deremaxx is a good pain reliever, but blood values should be monitored every few months to catch any internal changes.
Good luck with her, and keep us updated on test results :)