I have read that there's more serotonin in the "gut" than there is in the brain - NO IDEA how or if that relates (LOL).
Might be that you're exeoriencing a bit of a sugar high - or - that, depending on what you're eating.... hmm. Not sure.
Try looking at something like hypoglycemia - you may have a little sugar problem - or you may be waiting too long between meals and the your body is calming down from the stress after having been hungry.. something along those lines. More frequent smaller meals may even things out.