It would not be unheard of to have a heart attack and not realise.

Ask to have your thyroid function tested if it wasn't already. My mum suffered from Paroxysmal Atrial Tachycardia, and I am in the process of being assessed for this also, following some very scary palpatations, sharp chest pains etc in the last few months. I had to actually ask the doctors about this myself, as none of the 3 that I saw regarding the chest pains thought them of no consequence.

Sounds all very scary, but I am glad you are getting help and hopefully they can pin point exactly what the problem is.