About the heart attack feeling -- yeah, drinking too much caffeine can cause your heart to beat faster, and when you become aware of it (and start getting worried about it) it can bring on a panic attack...which has fun symptoms such as breathlessness, chest pain, trembling, rapid heartbeat, a sense of impending doom etc. So all things that could be seen as a heart attack. Thinking that it might be such ironically leads to the panic attack worsening! Caffeine doesn't need to be cut out completely, but cutting down would definitely help. Anxiety can generally manifest itself in different ways, too, including bodily pains and changes in diet/digestion problems.
If you want to get away from what you're writing for a while, but still want to carry on with the actual act of writing, writing (and how many times can I say writing in one sentence?) out worries or problems or just general feelings sometimes help. Just getting it out of your head and into some kind of coherent order can sometimes help...