Even a loose tooth that must be removed is not considered a
chatzitzah, since it is a natural part of the body. So long as you are able to clean your tooth properly, you may immerse in the mikveh without having the dentist check it first. In fact, this may be to your advantage since a temporary filling presents certain halachic questions (which may well be what you learned about).
There is a general halachic preference to delay dental treatment until one week after immersion. Of course, if you are in pain, you can go to the dentist sooner.
Refuah sheleimah!