i was on zoloft years ago for a major depressive episode. it took a few weeks for it to kick in but when it did i felt like anything was possible. i do remember feeling very tired at first though i wouldn't have known the cause since my depression was depleting me so terribly.
there were about two weeks that i couldn't get out of bed but again, that may have been the depression and the fact that i had started psychotherapy and my "lid" was opened up and all sorts of emotions were starting to flow out that were exhausting and very painful. stick with it, i hope it helps you the way it did me for the time i needed it. good luck!
first few days can be hell. give it a few weeks and see