I've a pretty good feeling that a declined ab crunch did it. It can make you a better ab cruncher but not improve core stability, if that's what you're going after. The exercises your chiro suggested are good. If you are training for a sport, you'll want to do even more specific exercises.

Also, before doing a deadlift, you should never stretch the back or the hamstrings, but rather do a set of light deadlifts before ramping up.