well, I work in psychiatry and I've learned and read a lot about this stuff (don't do this at home, you'll become a hypochonder!!)
Mood swings are "chemicals" in your brains that come and go. Endorfine makes you happy, if you don't have a lot of that "chemical" in your brain, you feel downish.
It's also normal, after having a few "feeling good" days, that you feel down, because that's how your brains are balancing out. You feel more tired and unhappy then.
It's been proved too.
People who take XTC, a drug that makes them happy for the moment, have a very bad feeling 1 or 2 days after the working of this drug stops. This is the reaction of the brain balancing out. There has been way too much endorfine in the brains (which came there unnaturally) so the body thinks there's still too much of that chemical. Brains stop making endorfines, and then you feel really downish. They call this dip "suicide tuesday"
(google it for more info) of just ask me
