Loose Teeth? Help!

Hi, sometimes I notice that my teeth move a little bit. I grind my teeth at night sometimes and I wore a night guard for a while. I wonder if either of these things might have caused the problem. My night guard stopped fitting the same way after a while because my teeth had moved. I only wore it for about 3 months and I thought it was strange that my teeth could have moved so much in such a short time period. I do know that in the past, however, I had orthodontic work done and then I stopped wearing my retainer after a while..... and when I tried it back on at one point it didn't fit right. Hope that sentence makes sense. Anyway, I suspected that my front teeth were a little loose a while ago and told my dentist. However, he tried to move them and told me that they were not.
So........ what is the problem and do you know if night guards can exacerbate loose teeth? The OTC ones have a warning re: don't use if you have loose teeth. Or could it be gum-related? What can I do about it, as well?
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