When I cry a lot I get fever, I don't understand this, someone could explain me?

Stress can cause it. Fever of unknown origin can happen though not commonly, under stressful conditions and at times seemingly randomly. It is possible that stress great enough to cause one to cry could be the stress that causes fever. A good medical evaluation is in order.
Crying fever. Interleukin (il-6) is a substance found in our bodies that trigger a fever warning of danger. Donovan et 2009 found that clinically anxious participants exhibited significantly lower levels of morning cortisol and significantly higher levels of il-6, independent of age, sex, and depressive symptoms. Crying is a catharsis and/or manifestation of anxiety possibly may increase risk for releasing il6.