Boy, can I relate.
I had the same sort of thing with my daughter.
Here is what I learned:
I needed to build her immune system stronger. I watched her sugar intake . Vitamins Minerals and Protein build the system, Sugar tears them down. What was the common denominator of the year? Halloween! Plus all the sugary things she was getting in her AM Cereal, and who knows what else.
So, when she was eating all the bad stuff, her immune system went south and she would get sick. I did a lot of checking on supplements to build immune system. ( I found the ones on the market.....even what we think are the better ones had artifical this and that, sugar and stuff that really shouldn't be in vitamins.) I bought her candy back from her, threw it out and with the money she bought a special Barbie she wanted. I went through the closet and got rid of a lot pre made foods, yes, even the Mac and cheese because of all the artifical ingredients. I did my research and put her on Shaklee supplements. (I can put you in touch with my person if you need one.) and "Voila"!
It made the entire families health better as a result.
Let me know if I can help in any way.