If i lie down, i can feel my heartbeat very forcefully pounding in back of my head where my neck meets the base of my skull. bp was 115/80. 26yrs old?

Likely no worries: Your normal blood pressure is the key. People may commonly be aware of their "heartbeat" in ears, head, neck, hands or abdomen- actaully the sensation of blood flowing. Since the heart sends blood everywhere- you can notice it anywhere. If it is a normal slow beat with normal blood pressure, you can rest easy. Don't wish it away- people who used to have a beating heart generally heart miss it.