Nissan 2003 2.5s, started shaking. Sometimes when I turn on the car, driving or at a stop. Why

Nissan Altima 2.5s 4cyl. I was driving fine, tuned off the car to go into a store, when I turn it back on I began to shake and as I was backing up, I then parked the car, turn it off and waited for a few minutes, turned it back on and the car was fine. Again, after running some errands, I started the car but this time when I turn it off and back on, it continued to shake, I drove off and later the shaking had stopped Later at a light the car begun to shake again. Changed spark plugs and coils and still continues to shake but Today it started shaking again this time stalling out completely and now won't start what could be the prob?