My clear retainer overheated, and is now tight fitting and difficult to remove and put on.

Looks like the heat may well  have distorted your retainer. If you can still wear it with not much discomfort,  try to do so to prevent “ drifting “ and movement of your teeth.

 As soon as you get back see your orthodontist / dentist to consider making a new set of retainers. Again having a spare as mentioned may be worth considering.

I agree with Dr Grinbaum's answer for your question. In addition I would suggest that at your next appointment that you get two retainers made. This will give you the option either to rotate the wear of both (alternate nights so that neither gets overused or distorted). Alternatively you can wear one and keep the other one for emergencies or to put on every three months to make sure that your teeth are in a stable position and are not continuing to move.
In terms of care, I guess you understand the problem with heating them or washing them in hot water. A weak solution of white wine vinegar diluted in cold water once a week is a good way to generally keep your retainers looking good.

Good luck and kind regards - 

Thomas Dey, BDS, MSc

Hi there, it may be that your clear retainer shrunk due to overheating. If the retainer still fits and sits completely on your teeth, I would recommend to continue using for now. The ideal thing to do is to see your local orthodontist and he/she can determine if the retainer is still a good fit to properly retain your teeth.
I hope this helps,
Dr. Karin Grinbaum