Is it normal for your teeth to feel very loose while using Invisalign?

Increase in teeth mobility is common during orthodontic movement of the teeth.  This is due to the remodeling of the bone around the teeth being moved.  Once the treatment is complete the teeth will normally tighten back.  However, if the mobility is concerning you, I would recommend bringing this up to your orthodontist and dentist.  There are instances when treatment must be postponed are terminated all together if there are issues with the periodontal (bone and gum tissue) health.  

Its normal for a teeth to feel slightly mobile after undergoing a lot of movement. Once the movement is completed the bone around it should tighten up and the tooth will feel a lot more solid. If you are concerned you should check with your dentist to make sure the tooth is not being moved too fest.