I was diagnosed with this a good few years ago, I also have arthritis, so not sure where stiffness and aches are coming from the most, I take Naproxen and Co Codomal, which I would rather not, but was wondering is there any medication for fibromyalgia so maybe I could try that and see if it helps just taking that.
Yes, "The most commonly encountered side effects of amitriptyline include weight gain, gastrointestinal symptoms like constipation, xerostomia, dizziness, headache, and somnolence."
I usually ignore the leaflet that comes with drugs and only worry if there are serious problems.
Is your gp managing it or a rheumatologist, if it's just your GP it might be worth asking for a referral in case there is anything they can add to the mix.