Jason's A Slave 4 Britney!

Yesterday, Brit and her boy Jason rolled up to Fred Segal's Mauro cafe in her brand new Beemer -- a b'day present for her 28th.

Britney, who smacked Jason's ass in the way in (@ the :22 mark), seemed to be completely in control of the relationship, even screaming at her man for leaving her alone to take a phone call!

An X17 photographer tells us: "Jason went outside to take a business call on his cell, and you could tell when he left her alone that Britney was really upset. She had the look on her face, like, ‘what the hell do you think you’re doing?’ He seems a bit like a slave to her.”

Doesn't look like Jason minds too much -- he's not tryin' to hide it!