Amoxicillin won't hurt her, but unless there is a skin infection present, it won't help either. The 250 mg capsule would be the dose for a 25 pound dog, so the dose is a little high for your dog.
The Silvedene ointment is an antibiotic ointment generally is used for burns rather than skin irritation. While it is not harmful, it probably isn't doing much to soothe the irritation.
The Desitin acts as a barrier to water (which is why it helps prevent diaper rash) and thus isn't that helpful for these types of skin irritations either.
The Preparation H contains an ingredient to constrict blood vessels. I really can't think of any situation when I would use this on an animal.
I would bathe the area (maybe get her to sit in the tub for 10 - 15 minutes) and then pat dry gently and apply a hydrocortisone ointment. This is an anti-inflammatory ointment that you can get at the drug store and it should help soothe the area. You can repeat the bathing and application of the ointment 2 to 3 times a day. I think you will find it more effective than the products you have used previously. Also, if she is very uncomfortable, your vet can prescribe an oral medication that can give her some relief also.
I hope this answers your question. If so, please click "Accept." If not or if you have additional questions, please let me know.
