I'm on the opposite side of the spectrum as you. I menstruate at the drop of a hat...so to speak :(
However I know that the majority of women have no signs of menstruation when still breastfeeding even if it's just at night. That doesn't mean that you are not ovulating though. So keep trying, eventually you will conceive.
How much do you weigh?
What is your daily diet like? Examples of food you eat on a regular basis?
You may want to look into getting your hormone levels tested, you can do this easily with saliva tests.