My DDs were the same, constant illness for years, low school attendance. Again always put down to childhood illnesses. Oldest was under the paediatrician for years and nothing ever to explain it, just unlucky.
Then my oldest was diagnosed with a form of under active thyroid at 11yo, her thyroid wasn’t actually working which was delaying puberty for her. It was only discovered by chance when trying to figure out something else that was happening to her. It means her antibodies are attacking her thyroid rather than her antibodies attacking infections.
I have also got several autoimmune conditions and again, spent my life having illness after illness, I’m constantly getting I need to eat healthier, take this and that vitimin. But really the last thing I need to do is boost my immune system. I was on immune suppressants for a while but I couldn’t handle the side effects. I’m taking a range of medications now which help manage my conditions.
Like your DD during covid was the healthiest time for everyone in our house, with the exception of actually getting covid which we for 6 times during lockdown, we as a family had never been healthier.
We are incredibly careful, hand washing, avoiding germs ( soft play is a killer for me), lots of basic hygiene around home and out and about.
The last thing I want is my DDs to miss out though, so we still attend events and do things, we just are extra vigilant. I will also avoid things in the run up to an important event so I’m not unwell. My oldest DD is the same to, she’s given the option as she’s at an age she can decide if she wants to risk exposing herself to anything then potentially missing something else.
My younger children were also very sickly when younger, slowly growing out of it, but this winter has been hard, we’ve had the attendance letters from school (which I ignore).
My advice, keep a diary so you can see trends and patterns and if you can maybe look at trying private. Most GPs just do generic testing and don’t always know what they are looking for ( words my consultants have said). Autoimmune conditions are very common and Drs are growing more knowledgeable about them.