Just purchased a used (60k) CX5 GT. Had it about a week with no issues at all. Today as I was pulling out of a parking lot, the car lagged/jerked for a split second and the engine light came on. Made it home and it threw P3400. There were no other issues of jerking, sounds, loss of acceleration and so forth. Oil level is fine and clean. Gas was very low when it happened. According to my VIN the recall on Cylinder Deactivation was completed. Does this fall under the Service Alert and am I looking at a broken rocker arm possible new engine?

 

Including what you stated if the recall was completed that shouldn't be your issue and there are other multiple things that can cause this.
Did you purchase in a private sale or from a dealer?
If from a dealer take it back and have them find it and fix it (you should have some type of warranty especially after only a week).
If from a private sale, your best route is to get to a Mazda shop and let them find out what is actually wrong.