skin or horse back problems

Question
I just purchased an 8yrs old thoroughbred(Kentucky-Bred Throroughbred).  I rode her 6 months before purchasing her.  She is a mare 15.2 hands.  We are two beginner/ intermediate riders that ride her in the ring and I also ride on trails.  Over the last couple of rides, I find that when applying pressure on her back, she collapses the back and starts to move around not wanting to be touched.  On trail on the canter, she would really keep her head down.  Now that can as a result of my hands moving too much maybe.  I am told she has had skin (skin is very sensitive) and oreness of the back in the past.  So, how do you know when a horse has back pain Vs skin pain?  How do you know when Anti-inflammatories should be used and/or acupuncture?  Is a good idea to have acupuncture once a month for example?  Is there a good book I can purchase to help increase my knowledge in regards to riding and back pain.  What do you recommend for muscle (back) stiffness?

Thanks

Answer
first of all,make sure your saddle fits her properly

if she is not showing some typw of scurf over the area, it is unlikely to be a skin problem.

use your fingers to press down hard over the back from just behind the saddle all the way to her tail. if you can get a strong reaction to pressure over the kidney area, it could be a kidney problem.

you will probably need your vet to look her over and suggest the proper remedy