I have major depression and am on zoloft (sertraline). every once in awhile, i go through a period of intense loss of interest and can't even get out of bed. why?

Sounds like break-: through symptoms; that is, the depression is occasionally still manifesting. Sounds like the Zoloft (sertraline) is helping, perhaps not enough or still needs a different dose or additional medication to boost its efficiency. And, are you in therapy? Studies are clear that therapy works just as well as meds in most cases, and adds efficacy to meds when combined. See your psychiatrist to start.

Major Depression: I understand your distress. “intense loss of interest and can't even get out of bed” are symptoms of Depression.. Zoloft (sertraline) is to treat Depression. At times, it can worsen depression. It is likely that depression due to painful feelings of hurt, resentment, anger, shame or guilt have not been taken care of. See Psychiatrist discuss, benefits and side effects of Zoloft (sertraline) and other medications; Therapy.