Outside stress can definitely influence moods. For example I'm not even trying to figure out a mood state for myself right now because I have so much outside stress and anxiety that I don't think it's even necessary. If I were to try to guess I would say I'm in a minor depressed/mixed state. Anyway even when medication stabilizes you outside stress can take its toll. Going back to work is a major stressor.

Ride it out - don't panic. Last year I went back to work straight off a major mixed episode complete with psychosis that landed me in the hospital. I was there for ten days and went straight back to work with only the long weekend. It was difficult for the first few weeks even though I had stabilized on my medications because of the stress. But it evened out eventually. Don't panic and don't read too much into the mood changes for right now. Just try your best to use stress relief techniques like breathing at work and after work until you get back into the routine. If you are still having trouble in a couple of weeks, share with your pdoc your concerns.

Give yourself a break though. Starting back at work is hard!