I get that lethargic feeling when I study too.
Look I don't know you in real life, so I can't tell your individual circumstances, but I'll talk in general. It's important to try to separate the normal 'down' feeling, what some may call being 'depressed' with Clinical Depression. If it's only one incident, it's very unlikely that you actually have Clinical Depression (particularly the way you seem to describe it, I don't think it is, but again, I don't know you in real life). Indeed it is very normal for mood to dip slightly up and down each day. If it is the case however that feelings such as a depressed mood severe lack of motivation, loss of appetite are persistent (for example, more thank 3 weeks continuously), then it is a serious health problem and you should seriously contact either a psychologist, or your family GP.