Before leading cops on a city-wide manhunt, Boston bombing suspect Dzhokhar Tsarnaev was busy partying it up with friends in a college dorm room ... this just two days after his deadly attack on the Boston Marathon.

The bombs went off on Monday and while law enforcement pored over photos searching for a lead, Tsarnaev went back to his dorm at UMass Dartmouth and acted like a regular non-terrorist college kid, according to the Boston Globe.

He reportedly slept in his dorm, went to the gym and then partied Wednesday night with pals from his intramural soccer team. A student source told the paper, "He was just relaxed."

The 19-year-old even tweeted before the shin-dig ... "I'm a stress-free kind of guy."

But by Friday, Tsarnaev's face was plastered over every media outlet, the UMass campus went into lockdown mode, his brother -- the other bombing suspect -- had been shot dead and Tsarnaev was finally captured ... after an intense stand-off with police, which left him wounded.