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It could well be an allergic reaction. Pit bulls on a whole tend to have problems with allergies so it isn't a surprise that she tend to have incidents. What I would do is give her a little benadryl and see if that doesn't take care of the problem. Benadryl can be given at a dosage of 1-2mg per pound every 8 hours.
The problem with allergies is that they can cause rashes which in turn causes itchiness and the scratching can lead to an even lowered immune system which can allow the staph and yeast normally present on the skin to multiply unchecked which would then be a yeast or staph dermatitis. Those can cause itching as well. A staph dermatitis also tends to resemble a rash as well.
You can read more on allergies here:
If this was my girl, I'd give her the appropriate dosage of benadryl and see if you don't notice some improvement. Now if her rashes usually go away after a day or so, you might not know for sure if is the medication helping, but if you give the medication and do not see improvement, you would want to have her seen and check for a staph dermatitis
This doctor has a different approach for minor skin dermatitis that you might find helpful.
I would continue to use the shampoo but remember that too frequent bathing can remove the natural oils on a dogs skin and coat as well.
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