You aren't imagining the mood symptoms! Pred can cause all of the above.

I just hate hearing of people taking prednisone in the evening! ICK! ACK! Pred for AI stuff is best taken at the time of day when the body naturally makes adrenal hormones which is 5-8 am. Most take their dose as early as possible in the morning so it can mimic the body's pattern and so the bulk of the side effects are settled in the evening so you can sleep.

Forty mgs is a pretty hefty dose, and it's possible that you are temporarily feeling some increase in symptoms. That is common, and will probably lessen soon, but please be careful and watch the effects and act accordingly.

For now, try to be easy on yourself and when you feel you are being snippy or about to "boil over" just mentally step back and give yourself a moment to gather some control, although it does feel good to let loose (but only while you are doing it--the regret is immediate!)

Good luck!