Are you eating enough protein at breakfast and lunch? Also, are you drinking lots of water through the day? When I don't do these things, I have a major 2 p.m. slump.
I do most of my cardio workouts in the morning before work, but they don't really seem to affect my afternoons (they definitely give me more energy in the mornings). If I have a pasta lunch without protein, it doesn't matter whether I worked out in the morning - I'll be dragging my tail and fighting to stay awake by 2:00.
Stebby