Hmm, this seems to go directly against the way in which chocolate functions in the body -- it contains tryptophan, which helps make serotonin, a neurotransmitter that makes you feel great
!! Maybe if you had eaten a load of chocolatey goodness, the 'bad mood' could be viewed as a crash after an intense sugar rush, but you mentioned only having a bite here and there. It's not crazy to think the two are related in that not every human being reacts the same way science has predicted it would. Could the resultant bad mood be a psychological reaction instead of a physiological one -- that is, sounds like you never liked chocolate all that much anyway, then you've treated yourself to it, and it hasn't been very rewarding to you? If so, maybe choose something you really appreciate as a special treat, and savor every bite.