Could be a million different things really, but the first thing that comes to my mind is arthritis. Yes, she is on the young side, but arthritis can definitely hit at any age. If she was exercised a lot before and isn't as much now, that can have a detrimental affect on an arthritic horse.
Depression doesn't account for change in movement, so I wouldn't look too hard down that road.
How soon after you got her did you feel that her energy level was decreasing? Maybe a change in feed affected that. It may or may not be related to the soreness.
Try to locate where she is sore. Poke and prod her all over and watch her reactions as you go along. Somewhere she will likely tense, flinch, flick her tail... do something to give you a starting point.
When did the vet say she's OK? Now, or when you got her? A horse that has foundered once, is often genetically prone to founder and needs to be watched carefully. If she's foundering again, you need to deal with it pronto.