Hey GingerB -
It's posture and tensing as much as anything most of the time. If you're gripping the bars like grim death, on account of going at faster speeds and all that, it's quite possible it's going to have an effect in the neck after a while.
Look out for tensing the shoulders too - do that for an hour or so, and you'll definitely have neck-ache.
Try rocking your head from side to side, rotate it around to get cricks out of it, and make sure you flex it and your shoulders plenty when you stop, and before you get on in the first place.
You'll get used to it after a while, relax more, and have fewer problems. If you're worried about neck-ache, you'll tense up more and make it worse!