The "Jackass" star was hospitalized Wednesday to treat what he says is edema ... swelling in the body caused by blood vessels leaking fluid into the tissue. Bam wrote in a since-deleted Instagram post that he was in the hospital for only a few hours to treat "either reaction to new medicine or detoxing from i guess a bit of alchohol [sic]." Bam's already back in rehab.

TMZ broke the story ... sources close to Bam told us he put himself back on the road to recovery last week when he voluntarily entered a new rehab facility. He had been at an L.A. bar the night before checking into rehab. He had recently bailed on a treatment facility ... the second time he's done so this month.