Why is shrimp rubbery after reheating? How can I prevent this? I don't frequently use shrimp in my dishes (once a month at most) because I tend to shop my proteins based off what is the best deal and I'm not usually interested in shrimp unless it is large (at least 15-20 count) and under $7/lb.

Whether I make stir fry, fajitas, pasta, or anything else I can't figure out how to reheat it and have it not be rubbery.  Microwave is the worst culprit (even a few seconds in the microwave will ruin the shrimp for me), but even reheating on the stove changes the consistency.  When I eat it cold out of the fridge it is fine (other than being cold).

What makes the shrimp rubbery after reheating?  How can I reheat it and prevent (or at least reduce this)?  FWIW, this happens whether I saute, boil, or bake the shrimp for original preparation.
I agree with @Joe. It sounds like you are heating too much and overcooking the shrimp. Once things like shrimp are overcooked I am not sure that there is a way to make them tender other than to cook for a long time, similar to slow cooking. With squid the rule for cooking is less than 2 min or more than 20 min to ensure tenderness. 

I think your best bet is to heat slowly and monitor as much as possible to make sure it is warmed but not heated fast so that it overcooks.

In the microwave I would spread the shrimp in a single layer on a plate and using a low power setting, heat in perhaps 20-30 second increments, testing for warmth after each step. You could play around with time and power settings to see what works best for you. The key is to make sure the shrimp do not get too hot, so make sure you turn them and shuffle positions on the plates.

You could also try heating in an oven, covered with foil at around 80-100 C for about 10 min (this is a guess on time, it will depend on the size of the shrimp).