what's wrong with my 2001 Pontiac Grand Prix

when I start up my car and drive it's fine until I hit
around 40 mph. the rpm also goes up really fast.
once I hit 40 mph my car loses power and slowly
slows down. if I hit the brake it won't accelerate. it's
a one shot kind of thing. it's a v6 the tranny fluid is
fine. the problem first happened today when I
picked up my girlfriend, I started up the car and
started driving, and when I went to put my phone
down I hit the shifter and my transmission went into
3rd gear( I was going 35mph) I stopped the car and
put it back in drive and when I hit 40 mph I lost
power.