Im very used to eat prawns with shells on. I peel the prawn and eat it. I have never had any problems! for the?

had any problems what so ever. But the last two times i have also "sucked" out the head. Both times i have been really sick a few hours after i have eaten it. And then headache. Is this just a coincident or is it simply "something" in the prawns head which makes me sick?