good job on bfing your baby!
bf-ed babies are rarely constipated, since the texture is watery, it's not constipation. as for amount, some bf-ed babies poo once every 4-5 days, other poo after every feed, as long as baby is gaining weight, it should be alright.
foremilk imbalance results in frothy greenish poo but since you are latching for 20-40 mins, not likely to be the problem.
i pretty much ate everything and anything when I was nursing, it did not really affect the baby although i did think chocolate made him a little gassy. are you eating any antibiotics?
as for nappy rash, that's common so be sure to put on some barrier cream during nappy changes.