I have mono, again, now what?

I had mono when I was 15, I am now 21 and tested positive for the EBV (or something) virus... The doctor told me it is possible to have it again, but she tested my mono antibodies when I was 18 and my body tested positive for having the antibodies. I just don't understand how it is possible if I had the antibodies to get mono again, or maybe my body wasn't completely covered? Anyway what does this mean? I know what I have to do, Rest and Relaxation, but I am worried now that something in my body isn't right because I have got mono, again!