08 Chev Cobalt transmission problem?

Hi there. Recently had a scare with my 08 Chevrolet Cobalt 112k miles where upon reaching about 45mph the old girls pedal went soft and the rpms would fly up with just a gentle touch to 4-5k when normally the lady would shift gears at roughly 3k rpm. I also would get no more power out of her, even if I slowed down (it would not want to shift down either it seemed?) then all of a sudden it lurched into gear. Now I’ve owned her since 30k miles and she is 99% highway driving commuter car (I live in Texas, its a long way to anything). Obviously it is an automatic, and after the scare I went and had the transmission fluid changed but then again today the same thing happened in exactly the same way, loss of power at 45mph. What am I looking at here? How expensive is this? I am a college student living paycheck to paycheck, so is this something I can diagnose and fix myself? I have experience in building/repairing motorcycles… Thanks for your time!