Hi Lucy and welcome to the forum.
Cut out carbs from grains including ALL sources of gluten (look for online list which includes shampoo, play- dough, cosmetics, many salad dressings and other products you don't suspect), sweets, sugars including artificial, dairy and give it a go for a few weeks.
You might be surprised!
Check your thyroid function (not by standard blood testing) but by doing
Dr. Barnes Basal Temperature Test. Just follow the online instructions precisely.
If you have chronic stress-beware of hidden stress- then your adrenals might be suffering i.e low adrenal function.
Look online for an adrenal fatigue questionnaire to gauge this.
If you prefer, I have one I could pm you; just let me know.
Other underlying causative factors could be pathogenic infectious conditions like pathogenic mycoplasma ( search under Dr. Garth Nicolson here in medhelp for details), metabolic abnormalities, candida, neurotransmitter dysregulation (only assessment questionnaires are usually available to the patient for this), PSTD-like conditions from traumatic past events (sometimes not even in memory-hypnotherapy can help with this).
Note: Most of these fall outside the scope of conventional medicine practice, as they are time consuming and "established medical tests" do not reveal anything significant, so the patient is usually left in limbo,
given only maintenance drugs for symptom management.
If you need any details in any of the above, please let me know, however,
my comments are not intended to replace medical advice.