well, it has happened. i just recently met a woman who's dds are 11 months apart, and she was nursing!
maybe you are super-fertile, and maybe your uterus is just being weird. i don't have much advice to give you except that you may want to invest in some OPKs and pregnancy tests if you guys are going to have sex without any form of contraception. charting is great and all, but nursing makes it challenging to say the least, so perhaps you need the OPKs to help you there.