There are a lot of things that can be contributing to that...
My first concern is if you're eating enough. If you're starving all day, you're going to be miserable. I certainly get mighty testy when I'm hungry... moreso now than ever before.
What are you eating? If your blood sugar drops too low, you'll find yourself getting cranky and irritable. My DF is the KING of this as he is a type II diabetic. I've gotten to the point now that when he gets snappy I tell him to check his sugars.
So you may want to remember to eat enough carbs to keep you going through the day. Just don't overdo it!