because of your position on your bike your shoulders are slumped so the weight of your head is more evidently felt on your shoulders than when it was straight all you need is ICE which is a kind of muscle relaxer that you rub to the affected area or any other pharmaceutical that helps muscles also go get a checkup with a doctor to make sure nothing serious is wrong otherwise and be careful in the future
It could be the bike is too large, the handle bars are too low or as simple as the saddle is pointed down in the front too much and causing you to use your arms and shoulders to stay in the saddle.
