Laughing gas depletes vitamin B12, and a deficiency of the vitamin can produce neurological symptoms like psychosis. In people who are already deficient in B12, the use of laughing gas could lead to issues.That could have something to do with it.
Other than that, how have you slept recently? If you haven't been getting much sleep before you had the teeth removed because of the pain, you might be chronically sleep deprived.
Hypnogogia is a state between being awake and asleep where your brain begins to shut down control of your muscles and you begin to lose consciousness. Usually this happens quickly for most people. When you haven't slept properly for a while, you will notice hypnogogia more readily and it can feel like you're suddenly sinking or becoming light headed. You can also hear or see strange things because you're partially asleep and your brain is acting like it's dreaming.
I'd suggest you see your doctor and have your B12 tested and ask for his advice. Barring anything being physically wrong, you might want to turn on a TV or music whilst your falling asleep to distract you from the weird sensations and sounds so that you can sleep better. Once you've had a long restful sleep, that might be all you need for it to stop.