Ativan has a half-life of 9-16 hours according to its wiki page. The depressive part could be the Ativan working, perhaps too high a dosage, and the return to fight or flight when it wears off. If you've been on it longer than two weeks it's possible you're experiencing interdose withdrawal symptoms or have developed a significan degree of tolerance, though dependance is more typical after four or more weeks of administration. It varies between individuals.

Personally I'm not a fan of benzos and would recommend any number of alternative solutions to them. I think they cause as many problems as they solve while predisposing people with mental illness to relapses once dependance has set in.