I have night sweats and then get cold, my sheets, hair, everuthing is wet. What could this be?

Many causes. Depending on your age, sex and health, many things come to mind. Hot flashes of menopause can do this. If not the case, undiagnosed cancers, tb, thyroid disease and other infecitons can be the cause. You should get in for and exam and testing. This could be benign or very very serious. Best to you.
See below. Agree with Dr. P. However, you may wish to try reducing the amount of covers you use while sleeping. Improving air circulation in the room, with a fan, may help.