What time of day do you take your AI? I started seeing symptoms about 6 weeks after taking Letrozole. Muscle aches in my arms and joint pain in my hands and lower back. I first tried taking it at night, but have switched to the AM and the night pains are much better. My worst side effect is the uro-gyno atrophy (drying and thinning of vaginal and urinary tract tissue). I had 3 major urinary tract infections in four months and very painful intercourse. Have tried the Bonafide vaginal inserts, but not very helpful. I have found that staying active (walking 2-3 miles in the neighborhood 3-4 times per week), stretching and yoga, and staying hydrated have helped tremendously with the stiffness and back pain. Still have some mild hot flashes, but not as bad as menopause (I'm 64 now). I have been on letrozole about 15 months and plan to continue if the side effects stay status quo.
Also for those with brain fog, have you had your thyroid, D3, and/or B12 levels checked? A year before my BC diagnosis, I was having terrible brain fog, digestion issues, and neuropathy in my right leg. Finally went to the doctor and found I was very low on both vitamin levels and was hypothyroid. Once I started taking D3 and B12, my brain fog lifted and I was back to my old self. I started on probiotics and changed my diet to gluten-free, dairy-free, and soy-free and my digestion issues and thyroid levels are better, plus I lost 45 lbs! Definitely worth checking into!