I tend to have a very low BMI and nurse for what seems like forever! And yes, both greatly affect my fertility. In my experience with working with breastfeeding women for 9 years now, women who do have lower BMIs, tend to have AF stay away longer OR have irregular cycles to the point where fertility is still compromised. Often gaining weight shifts the pattern where a decrease in nursing is not needed. Or some though, only decreasing nursing will help, I have only encouraged one mom who did have to fully wean. Ok, next is a time period. Having a 20 month old, lower BMI, and breast-feeding with no AF is normal. Average return of period with extended bfing is around 14 months, you are going to extend that for being skinny. I have had really thin moms go easily beyond the 2 year mark with no AF and when it did return, they still where nor "fertile", meaning the cycles were such that no pg was possible.
This is an area of interest for me because I have spent the last almost 10 years nursing and have had 4 children in that time without ever reducing my nursing with a lower BMI. It was not always easy. I never studied it formally but paid attention to the hundreds of moms I have worked with over the years and the patterns I noticed. My own stats are: DD1 was 2.5 years old when I finally gave up and induced AF back. I had already gained 12 lbs by then but was still considered under BMI. My LP was way too short to sustain a pg, I spend the next 6+ months working with herbalists to lengthen it with herbs. I finally got pg when she was 3 year old. DD2 was 18 month old when AF returned, I was slightly under my weight when I got pg with her. I immediately started my herbs again and got pg 3 months later. DS1 was 8 months old when AF returned. This is the most I had ever weighed non-pg though (at this point) and he was autistic and not a big nurser. Even though AF was back, it was irregular, despite me not really preventing, I did not get pg for another year until he was 18 months old and ironically the month I WAS preventing because by then I knew something was developmentally off with him. Figures! DS2 is 12 months and I weigh more then I did when I got pg with him, still an average BMI though. I don't know the actual number but I am 5'5 and 120 currently when my old weight used be to between 98-110 with the first two kids. And no AF yet. But I am pretty sure I ovulated this week, if I didn't then it is a really good warning that I am about to soon. Which I am glad for because #5 IS NOT HAPPENING EVER.