Well, in that case, I would ask your doctor about trying clomid or something similar :-) Good luck!
I would not mind multiples. I just say one last time as in after this pregnancy we are taking a break for a few years. :) I actually have always wanted twins. Lol I know I'm weird.
Based on your history of not menstruating on a regular basis, and therefore not ovulating (except for your regular cycles), I would suggest you ask your doctor about an ovulation inducing medication, like clomid, to make sure you ovulate and increase your chances. The only problem with fertility meds is that there is a chance you release more than one egg, and since you're looking for one last child, I'm not sure if you're willing to take the chance of possibly conceiving more than one. Best of luck with everything!