No, honey you're not being silly to think about things like that. The reality is that the dental x-rays have very low radiation and usually they will cover your abdomen with a lead apron to minimize your exposure. They are also very concentrated and are aimed directly at your teeth. I have been told (don't know if its true) that the 3d ultrasounds are equal to about 10 x-rays so there is very little risk to you from having your dental exam. There is a risk however to not taking care of your oral health- oral infections can cause several problems. Besides, many years ago they used to take x-rays of pregnant women as tools to diagnose a pregnancy before ultrasound!

I think you are probably pretty safe. Try not to worry too much