I don't think it's related to having bipolar, I know many people who don't experience any increased incidents.
I do think if one is having continued stress and/or less sleep, also poor nutrition, that these factors will be great contributors to over taxing the immune system.
There has been and still ongoing research about stress and it's negative effects on the immune system of humans and even animals/pets. Another thing that doesn't help, is having hypothyroidism, it interfers with many things. It may help to take note of your eating and sleeping habits, maybe you can find if you are not getting enough quality sleep and/or proper nutriton
in your daily diet?
Just my 2 cents.
Meanwhile, I'm sending you well wishes for good health and inner peace.
Take care,
DE