I would first check the tranny fluid (you probably have already).

Is your check engine light on? Also, when was the last tranny service? Could just need a nice flush and fill.

ever notice that more problems show up after your car gets serviced? i'd get an inspection from a different and trusted mechanic to see if anything was tampered with. heck , the dealership blew out all of my bulbs in my car for a tierod replacement and wouldnt fess up to it. you never know....