Mononucleosis is caused by the Epstein Barr virus and is common in adolescents and young adults. The symptoms could be exhaustion, feeling tired, feverish, have a sore throat, etc.
Mono proceeds in three phases. First, a prodrome lasting 1-2 weeks with few if any symptoms. Second, an acute phase lasting 2 to 6 weeks during which the individual may be very sick with fevers, swollen glands, severe sore throat, and exhaustion.
If you have had MONO in the past you could still test positive for the test.