Adopted boston terrier rubs against furniture?

Hi all, I recently adopted a Boston Terrier and he’s been rubbing up against furniture and people like he’s trying to scratch an itch….this happens pretty frequently. I gave him a bath as soon as I got him home b/c he smelled bad, so I’m thinking it may be a reaction to the shampoo. (there were no fleas in the bathwater, so I’ve ruled that out)I’ve also done some research and I see BT’s are susceptible to food allergies. The rescue org. I got him from was feeding him Gravy Train (so have I), which doesn’t look like quality food. So now I’m thinking it could be food allergies. Any ideas on whats causing this and what to do about it?

P.S. I’m a first time dog owner, so all of this is new to me.