you could have got mono from your fiance a lonngg time ago. Mono symptoms often take 3-4 months to even start bothering your body. You could have gotten mono 4 months ago, but are just now starting to show symptoms. Ask your fiance if he/she has ever had mono before- even in their childhood. Mono stays with you forever, but you dont show symptoms, its just in your system.
Also, you could have shared a drink with someone-it spreads that way too
I hope you feel better, get lots and lots of rest and drink plenty of fluids!