I missed periods all the time because of being underweight, sick, and on IV abx all the time. I would normally only miss one month at a time though....that's why I didn't realize that I was pregnant when I was. I missed my period in June 2005 and didn't think anything of it because I always did and then missed it again in July and thought that something may be up but I didn't think honestly that I was pregnant. I took a pregnancy test just so I could rule that out before I went to the docs and ummmm I couldn't rule it out because I was pregnant! I thought that it was a dream because I was at my sickest because I had already started the evaluation process for transplants!
Anyway, yeah I always missed here and there. Since the Tx though, I never miss and am always on time. It is also much heavier than it ever was and it's quite annoying!