Put retainer in after one month of not wearing, feels tight and made one of my teeth loose. Will that tooth tighten back up?

See orthodontist . Your teeth should not be loose at your age. See your orthodontist asap about this. The orthodontist knows you and your teeth and will be the best one to answer your question. Be honest about how much you have been wearing your retainer. It will help them know why your tooth is loose if they have all the information.
Likely... The tooth is likely loose because retainer is making it move back into position...This is how teeth moved with braces as well, but you didn't realize it since they were fixed to the wire. Regardless, you should follow up with your orthodontist to double check that the retainers fit ok and all is normal.
How sad. Retainer didn't change, your mouth did. Teeth trying to move back to their pre-treatment positions. Please call your orthodontist for an examination to see if an adjustment is necessary, then follow your orthodontist's retainer instructions carefully if you want to prevent relapse.
Orthodontist. Teeth should not be loose. Sadly, during the lack of wear, your teeth shifted. You may have issues with your root structure of that tooth that makes correcting now a risk. i would visit your orthodontist, have them take a pano of you, and see if they can recommend a course of action.