My bottom teeth have moved a little bit. would my dentist be able to make a new retainer to fix it?

Maybe: Movement of lower front teeth is the most common form of relapse after orthodontic treatment. This problem is usually prevented by wearing a retainer after active treatment is completed. It can be corrected, but may be beyond the scope of a 'retainer'.

Possibly: Depends upon how much movement and if there is room to move them back. Often the teeth around and opposite these also move. Unless there is extra room , all these will have to move as well. Check with the orthodontist.

Possibly yes: If the movement is minimal and your dentist offers this service, he/she can make you a new retainer (aligner) to correct the problem. Good luck.

Yes: Depends what you mean by "a little bit". If you have spaces or crowding totaling 3mm or less it could be corrected with a retainer. You may need to "slenderize" your teeth if you are crowded. This can be done painlessly by your dentist.

Sort-of: Retainers retain, they hold things where they are, they don't straighten them. If changes are very minimal, some correction may be obtained with a traditional retainer. If moderate, there are removable appliances that can address a few mm of change. If more severe, braces, tooth reshaping, and/or invisalign may be indicated. All appliances would require 24/7 wear to be effective, then retention.