Michael Phelps -- Out of Rehab ... Back with Miss California

Michael Phelps is finally out of rehab -- but it seems like he's already picked up an old habit ... Miss California.

Nearly two months after entering a treatment center following his second DUI arrest, Phelps spent his Thanksgiving with family members ... and on-and-off girlfriend Nicole Johnson (Miss California 2010).

Phelps and Johnson have been together ... and then not together ... and then together again ... since 2007. According to Johnson, they're officially back-on ... and even had a movie "date night" last week.

Phelps was joined at Thanksgiving by his mother, sister and other family members -- and it's interesting to note there was wine at the dinner table ... since Phelps is clearly struggling with alcohol.