I have bred shrimp and did my dissertation on ecdysis. How often they molt is dependent on how much they eat and how young they are. In the simplest short answer: When they are very small they could molt every few days. The shrimp eats and fills its existing shell then molts so it can grow. After it molts all the meat is used to make a new shell and this is when they are called "paper shells". They can be just a shell with very little meat in them. You should target feed them after a molt because they are vulnerable to predation when their shell is "paper". Then the more they eat the faster than can fill that shell and molt again. A lot of crabs and shrimp molt during the phases of the moon. Most will molt the three days following the full moon and a substantially less amount of them will molt during the new moon. I would assume that your shrimp molted during the last moon phase and then again four weeks later during the next moon cycle.