there are a few things to consider. how long has it been since you had mono? if you were sick for a long time you may have lost some muscle mass which takes time to regain. also consider changes to how much you now eat ( you mention diet but not quantity). a registered dietitian can help you troubleshoot changes to your diet you may not be aware of. sleep may also be a factor. i also recommend getting a physical and checking blood work etc. good luck!
i am unable to lose weight after mono. i got mono last fall as a college freshman (female) and gained 10lbs in a month. my diet and exercise has not changed; i eat healthy run and do circuits or intervals 5x/wk. i am now 15lbs heavier and very unhappy. i take iron supplements to treat fatigue but has not helped with weight loss. i was not diagnosed with hypothyroidism.