Heart beat acceleration

Occasionally i have episodes where my heart rate increases dramatically to the point that i feel light headed,dizzy ans nauseus. This usually occurs when i am relaxing,or about to go to sleep and often i am awoken from sleep because of it. It presents very similar to a panic attack but i am very sure this is not the cause due to when and how it occurs and how im feeling at the time. I am very afraid that i am going to have a heart attack while these episodes are occuring although i do not get chest pains while it is happening. Quite often prior to the episodes i have a soreness and stiffness of the back left side of my neck, i am unsure whether this is related or just a ongoing coincidence. I am very worried about this and would appreciate any advice you could give me. Thank you.