Thompson announced last spring that his career was over, after he hurt his neck playing for Northampton against Biarritz in January.
A World Cup-winner with England in 2003, he became coach of Brive.
But after medical advice, the 29-year-old is playing for his club again - and is not ruling out an England recall after figuring as a substitute for Brive against Connacht in the European Challenge Cup at the weekend.
"I didn't think I was going to be ready to play until December," he told Eurosport.
"I thought I might get on for 10 minutes - but I played for over an hour.
"I'd more or less accepted that I wasn't going to play again, so it's great to be back.
"I never retired from international rugby - and if the opportunity arises to play for England again I'd jump at it."