This is pretty normal as your teeth and the supporting structures are moving and different areas of your teeth are being exposed slightly that may not have been exposed before. I recommend a sensitivity toothpaste with potassium nitrate and fluoride. This should make that tooth sensitivity subside. Hope this helps!
They are both usually made out of similar materials but they do not achieve the same outcome. Fortunately, a night guard that is made of a clear plastic material and covers your entire arch of teeth can also serve as a retainer if it is made properly. An Invisalign retainer cannot serve the purpose of a nightguard, though. It is not thick enough to protect teeth and you will find that if grinding your teeth is something you commonly do, the Invisalign retainer will wear through in a short period of time. I think Invisalign is an incredible treatment and I would not let this stop you from pursuing it. Many times you can have a new nightguard made at the end of treatment on the lower and an invisalign retainer on the upper. Hope this helps!
