I found certain generics had additives that caused a reaction. Letrozole made by Accord gave me shooting pains across my shoulders and upper chest. The pills were pink so perhaps it was the colorant used as I've had issues with colorants in other meds and products. I found I had the least side-effects with Letrozole by Teva (light yellow pill). 2nd choice was Letrozole made by Breckenridge. I finished my 5 years in May so I don't know what pharma options are available now. Out of 5 pharmacies in my town, only 1 pharmacist would work with me and special order Letrozole from Teva (Pharmacist at local Walmart.) Most stores only work with their contracted supplier for generics. So you may need to call around if your current pharmacy will not change prescription to another manufacturer of Letrozole.
Hang in there and good luck.