Suddenly, my armpit hair and leg hair is growing in very patchy. Like, bald spots. But all of my thyroid bloodwork came back normal. Possible causes?

It depends/ nonspec. This seems like a nonspecific sign. It might depend on what products, if any, you might use to groom those areas. For example, if waxing or heat grooming instruments are used, the roots might be removed and the hair may take real long to or may never grow back. Stress might also be a reason, as might your gastroenteritis. If it gets worse, see your doctor. Good luck.