Eating lobster can cause diarrhea for a few different reasons, such as it being undercooked. It is extremely important that you cook your lobster all the way through and also make sure that it was dead before you cook it, because this will matter quite a bit. Because lobster tends to deteriorate rather quickly after it dies, you will need to make certain that your lobster has not been killed prior to cooking.
Make sure to always look out for lobster in the grocery store with a curled tail, because it will most likely indicate that it was not alive when being cooked. Those who are allergic to this particular type of seafood can also experience diarrhea, so you will need to keep that in mind when you are eating it. If you experience diarrhea, gas, or stomach cramping soon after eating lobster, it is highly recommended that you avoid doing so in the future.