Started Retainers Today Which Are Snug, Noticed a Loose Tooth?

It sounds Dr built a little over correction in to your retainer. Call the office and get an appoinitment to make sure things are correstly made and what is happening is planned

It is normal for your teeth to feel loose when you are undergoing active orthodontic treatment, such as Invisalign. Your retainer is a passive appliance, therefore no more tooth movement is occurring.  It is possible that the tooth is still mobile from the Invisalign treatment, but if you feel that it was not loose before, then I would recommend seeing your dentist for a quick exam.

When you complete your invisalign treatment the retainer should be almost identical to the last tray you were using.  If you wear the retainers properly your teeth should not move.  It is very common when you complete orthodontic therapy that some of your teeth may be loose.  If you are concerned speak with your dentist.  It is no something I would worry about.  

The Retainer should be very similar to the last aligner tray that you wore. It's unusual at this point in end of treatment for a tooth to be loose. Have your dentist Xray it & check for bone loss/periodontal disease.

Awfully hard to give a correct opinion with out seeing you clinically.  I highly suggest you visit your clinician.  If you think it is necessary you may consider a second opinion.