You need to have your prolactin levels checked. Also ask for a pituitary work up. Sometimes after childbirth you can get a non-cancerous tumor on your pituitary glad that either secretes hormones (like prolactin) or pushes on the gland and supresses other hormones. Be prepared to have to fight for these tests though.. from personal experience doctors don't like it when you tell them what you think is wrong. You should also ask to see an Endocrinologist, but even they might not want to help if you are not "textbook", because most just want to treat "easy" diabetes patients. Not trying to scare you, just telling you from personal experiance that its hard to find help these days unless you are a simple case. Hopefully you have a good doctor though.
With everything going on from the fatigue to the no periods, makes me advise you to see your doc. If you haven't been breastfeeding in 8 months, your period should have returned. Are you on birth control? Do you think there is anyway you can be pregnant. I was always unsuccessful getting my children to breastfeed, so I'm affraid I can't help you there. But for sure, make an appointment with your OB/GYN so that she can order some blood tests for you. You could possibly not be taking in enough iron, causing your tiredness.
Best of luck