lvfrogs is correct. If you acquired EBV (mono) at any time in your past, it will stay with you, dormant, for life. Given your recent positive test, I would say your EBV has re-activated. Basically, when this happens, you will probably experience all the same symptoms of mono, although there's no telling how long it may last. As far as why this re-activation happened, the most common sources are usually other health issues/concerns (especially those that weaken the immune sysmem) and stress. I hope this answers your questions, but don't be afraid to ask anything else!
What did your doctor say? If you had mono when you were young, then you have been carrying around the EBV since then. If you have had stress in your life recently or something traumatic or an operation, it might have restarted (for lack of a better word) your EBV. What are your symptoms?