pom/chihuahua mix skin conditions

Question
my 15 year old dog, 12 lbs.  is missing hair on rear sides and lower back and tail and legs. she is constantly biting at herself and has  high pitched whining. she sleeps for long periods at a time, but then wakes up starting the process again.  Her back legs seem stiff and she is very tender across lower back and sides. Skin is very pink and hot.  She also has bumps with black centers and has pus in them.  Her 4 lower front teeth have fallen out.  She is allergic to flea bites and has been on cortizone before and it helped before for the skin irrition, but the bumps and the whining and back leg problems  has us worried. She has also had blood in her stool at 2 different times and has a heart murmur. Could it just be her age. And could the whining be from pain or is it a habit?  

Answer
If she has been on steroids and they helped, you can use low dose steroids to keep the skin from itching so bad.  If there is an infection, that will need to be treated also.  As far as the other symptoms, your vet may need to run blood or do X-rays to see why she is whining.