Yep, quite common and is (in the main) only caused by a few things, which you can easily check. If you haven't already solved this of course, after all, it's been a few days.
1. Tyres themselves, i.e., wear. Once they start to wear in the centre of the tread, they can square off (that is to say, the bottom part of the tyre becomes flatter). What happens then is that the sides of the contact patch scrub on the road differently around the circumference of the tyre - as it rotates - and this causes the bar shimmy. Note that it's more likely to be the front tyre that's the culprit, but it could also be the rear. Examine / replace!
2. Steering head bearings (headstock bearings) either badly adjusted (too loose) or otherwise worn. This is more readily exhibited as a feeling of the bike not being happy about going in a straight line, or in other words, 'wandering'. But problems here can also cause the shimmy. Put the bike on the centre stand (does it have one??) sit on the back of the seat to get the front wheel clear of the ground and rotate the bars from left to right. If you feel a 'notch' somewhere then the bearings are knackered.
You need to get the problem addressed quickly though otherwise you could experience a tank slapper... which is NOT good!
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