Keep having reoccurring temperatures of 100-101 over the last few months daily. feel extremely fatigued, but normal cbc and esr. what could it be?

FUO: You're having fever of unknown origin. The list of possible causes is long and may make you stressed out. So please consult your physician for detailed history and complete physical examination and along with that some relevant test will get you the right diagnosis. Take it easy.

Many things: There are many reasons for people to have low grade fevers or frank fevers. They span infectious, inflammatory, hormonal, even cancers(though in a 19 year old that would be rather uncommon.) This is a very difficult diagnosis to make in a setting such as HealthTap, enough such that you may want to consider more formal consultation.