gear shifting help

I don't whats going on with my car.
my car is 1997 Galant, Auto.
One month back, there was noise from belt,
I showed to mechanic, and he repaced the pully i guess.
Then, it worked fine.
But, now when i drive, gear won't shift, meaning
when i put in drive, i feel it shift from 1-2 then car slows down.
even i press acclerator, (speeds up to 35- mps then slows )
And I tested with shifting low gear, my car takes speed
but driving all time in low gear in not good idea.
Can any one figure out what is the real problem.
At first i was thinking its due to cold but
Don;t know
I really appreciate help
Thanks