I thought from your question the bath was more recent.
If he is not eating well then this might be a problem with liver or kidneys particularly if this happened since you started the rimadyl as that medication can impact liver and kidney function.
Antibiotics often make a dog feel nauseous and puts them off their food too.
Spine issues show up often when a dog is lying down or trying to get up or down. And pain can bother you more when you are trying to rest than when you are active.
I think you need a blood workup done by the vet and perhaps x-rays to possibly determine what is wrong.
This could be from a tick carried disease. It might be from a problem with having swallowed a non food item that isn't passing through his system but is causing injury etc. It can be hard for the vet to detect what is wrong when the symptoms are vague but you can do more testing to see if you can pinpoint a problem.
I do think you need to see a vet about this and if your vet isn't much help you may want to see one who specializes in internal medicine.
http://www.acvim.org/Specialist/Search.aspx
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