I was on lithium for about 2 and a half years and found that it made my mood too low, but 3 weeks on a med isn't long enough for it to become a therapeutic level. Definitely let your pdoc know even if it's just leaving him or her a message.

Do you think your mood is cycling more because of the shift work. A previous career I had prior to my diagnosis had a brutal shift schedule and my mood was all over the place. It may be worth it to ask for a constant day shift schedule while you get your meds worked out.