Problems with transmission not shifting when driving faster than twenty five MPH?

A few weeks ago, while driving on the highway after getting out of heavy traffic, the car stopped shifting from 2nd to 3rd and the whole car would shake as the engine revved. I drove it around my neighborhood, and it shifts through the gears perfectly fine at low RPMs, but once I try to go above around 25 MPH it won't shift, and the whole car shakes. I tried doing a transmission fluid change, but it still does the same thing. Please help, I'm not sure what it could be.