It's a struggle when you are trying to keep up and the weight just wont stay on Alot of people are well meaning too, and probably say you look amazing after having a baby, when all you want to do is put on a few kilos.
A GF of mind has problems with her thyroid, she is super skinny too (she has the hypo one I think?) and only had her little girl the time as I did, it messes with her general well being too. Makes her grumpy and short tempered and light headed too.
The other thing to try, if you full blood test are ok, perhaps a naturopath? I recently had a friend who went to the GP, tired, exhausted and generally run down. Her blood tests all came back ok.
She went to the Naturopath and explained about the tests and how she was feeling, he said she did look tired and gave her a full look over. Asked her to take of her shoes so he could feel her feel.
He gave her some things to take and she is feeling better.
Might be worth looking into. Perhpas your GP could recommend a Naturopath and they could work together to get you back on track.
Being under weight would hinder AF coming back.

Take care lolli