Have you been evaluated for bipolar disorder? It's definitely premature to guess it's that based on this little evidence, but if you can, it could be worthwhile to take steps to see if you don't have a mild case. Cycling through highs and lows like that (in this case, from "sad" to "really really sad") could be a symptom.
I've also read that humans tend to cycle through mild changes in emotional potential throughout the day. Since your brain does stuff to make you sleepy as the day winds down, that drop in energy could perhaps manifest as a greater potential for lethargy and sorrow. What do you generally find yourself doing at that time?