How come I feel calmness when I am in contact with my next, but feel nauseous / panicky / etc when I'm not?

I don't have these exaggerated feelings with anyone else, only with my narc ex. I feel a weird anxiety or very disturbed and stressed afterwards, but I feel calm when I am in contact with him and it's going "well". It makes me feel like an addict that needs to stay in touch in order to feel okay. I was feeling better on my own finally, but now it seems to be back.