Hi from what I understand a full blood count picks up a lot of deficiencies, particularly iron, and I think the B12 as well.

My mum had a sore tongue due to iron deficiency and they detected that through a blood test so I think if bloods were normal you can stop worrying.

It's probably just a combination of stress and certain foods irritating it, these things unfortunately do take a while to settle once they are aggravated.

take care