I would tell them that you might be pregnant but that it is too early to test and see what they say.
I registered with a new dentist earlier this year and he wanted to do x-rays as a matter of course. He asked if I was pregnant and at the time there was a possibility, but I was less then a week post AF so no way of knowing for sure. He told me that as the x-rays were non-essential that they weren't worth the risk. He told me that the most dangerous time for an X-ray is during early pregnancy, and that there is no point doing them unless there is a very good reason.
I am sure that if you tell them that you may be PG, they can assess how necessary the x-ray is. They might also be able to reschedule you for a couple of weeks time so you will know if you are pregnant or not.