It'll take time for your neck to get used to riding.
When riding try not to crane your neck up, try looking up more with your eyes. Make sure your back is flat as much as possible, e.g. rotate your pelvis. When it's safe, occasionally look down at your seat post, chin to chest, to stretch out your neck. You can also try riding hand's free occasionally. To prevent strains, it's better to look under your arm than over your shoulder when checking traffic.
When your neck is stiff, try pulling your shoulders up to your ears and hold it for a few seconds. Rotate your head around like a top so that left ear, chin, right ear, then back of neck touch shoulder, chest, shoulder, and back. Then try looking up and back over your left shoulder, then the right.
Cold things work on sore necks, try putting a cold coke can on the back of your neck.
If it keeps bothering you, get someone to check your bike fit.