Mussel sore please help?

i was kyaking and when i came home the mussel in between my shoulder and my neck is super sore i tried putting a cold pack on it, then i put a heat pack, its still very sore..... what should i do?... whats wrong with it.... its not a growing pain.... on a scale of 1 - 10 its like a 7 or 8..... its a dull pain not sharp.... theres no physical marks