Can you help

I have a 2001 chevy camaro it has 64000 miles and is a 6 cylinder I was driving on the freeway at 65 then it stared acting funny so I pulled into a gas station to let it cool down but after I started it it wouldnt go over 40 so I drove it back home after a couple days I started hopeing that I would be normal but then it wouldnt go over 25 a couple people said it sound like the tranny and others said it was the catalac converter. So can you help me figure out the problem so I dont get ripped off by one of these repair shops