Diet can have a huge effect on the way you are feeling.
If you have days that seem worse for no apparent reason take a look at what you ate the day before.
When I am stressed I find wheat upsets me, I get really livery and nauseous. And dairy definitely used to make me feel anxious and trigger a panic attack if I was feeling fragile. I can feel palpitations very soon after eating anything with dairy in it, let alone plain cheese, yoghurt or milk on its own, so check ingredients lists too.