I would suggest not saying, "I'm easy", because usually that refers to you being willing to have sex with a lot of people, or you are easy to get (which is viewed as a negative thing). I think you might of meant "I'm easygoing". Now, this is usually used to describe your personality, meaning you are a calm and laid back person. "I'm calm" usually is used to describe how you are feeling, so it's more of a temporary statement.

'I'm easy' is a colloquial expression that means 'I don't mind'. For example: A: 'Do you want to eat at home or go to a restaurant?' B: 'I'm easy. You decide.' 'I'm calm' is not really an expression at all. It's a simple statement. For example, if you are about to take an exam, and someone asks how you are feeling, you might say 'I'm calm'. This means that you are not feeling nervous at the moment.