My mom age 51 has fever nearly 101 only at night, but normal during day all tests were taken nothing explained ?

Second opinion: Two things:First: check to see if your thermometer is accurate. Use it at the doctor's office and compare it with their's. Second: If it is accurate, then a fever of 101 at night is worth checking out. How much testing was done? Does she have any other symptoms? You can also get a second opinion if the first one did not give you an answer that makes sense to you.