I've read that it can become rubbery when overcooked but am not sure why it will sometimes also have a mealy texture. This has also happened to a lobster tail that I cooked in the past.
Probably overcooked or it was bad shrimp to begin with. To properly cook: Bring water to a boil. Add a tsp of salt and a tbsp of vinegar (you won't taste it...it helps keep the color and texture). Add shrimp. For medium shrimp, do not boil any longer an 4 MINUTES. Drain immediately and chill.
