hello and welcome to HCM forum,
nightgaurds are meant to gaurd you teeth from any kind of trauma,
custom -made nightgaurd fits
well and does not cause any movement of teeth,
but in your case,it seems OTC nightgaurd has caused slight drifting of teeth, it happens because of the ill fitting, and excessive pressure applied by it on the teeth.
therefore i would suggest you visit a dentist
soon, so that you have your own nightgaurd which is comfortable to wear and without any side effects.
till then do not wear any nightgaurd until your your dentist advises you .
do not worry, your teeth will be fine and back to their original position on its own.
i wish you good health.