Sean Combs isn't looking so puffy these days ... and not just because he changed his name from Puffy to Diddy ... he shed a bunch of pounds, but it's not a result of his time behind bars.

Diddy was looking thinner during his appearance in court Thursday and his face looked skinnier ... leading to lots of folks speculating he hasn't been eating in jail. Others thought the criminal case was finally getting to him.

However, sources familiar with the situation tell TMZ ... Diddy lost the weight BEFORE he was arrested by the feds last month, and it's the result of healthy living.

We're told Diddy was hunkering down in his Miami mansion for a long time this year as the feds were investigating him, and during this time he started a physical and mental health regimen to stay healthy.

Our sources say Diddy was exercising to remain in good physical shape ... though it's unclear how much weight he lost.

Frankly, photos from just before Diddy's arrest in New York City do appear to show signs of weight loss.