By mono I guess you refer to Infectious mononucleosis, more commonly known as glandular fever in other English-speaking countries. It occurs most commonly in adolescents and young adults, where it is characterized by fever, sore throat, muscle soreness, and fatigue. Infectious mononucleosis typically produces a mild illness in small children, and is often asymptomatic. Mononucleosis is predominantly caused by the Epstein-Barr virus (EBV), which infects B cells (B-lymphocytes), producing a reactive lymphocytosis predominantly consisting of atypical lymphocytes, a specific type of T-cell that gives the disease its name.
A diagnosis of Mono can be established through examination and tests. Though, the EBV virus may remain dormant for some time and resurface at a later date. Please confirm this from a doctor though it is clear that the disease may be resurfacing of a previous infection.
You can get more information at : http://en.wikipedia.org/wiki/Infectious_mononucleosis