“Stiffness in left hand?”

I'm a 26 year old female. I'm a very anxious person and recently I've been feeling stiffness in my left hand and fingers, especially on my thumb basis. It mostly feels in joints, but I do feel twitches in this hand and in other parts of my body as well. I've had an ultrasound which showed synovitis in my wrist, all the blood tests for rheumatologic diseases came ok. I've also been to a neurologist because I'm very scared of ALS. The doctor said it's not ALS. I also had ENMG in August because after training had weakness in the same hand. I should also note that these symptoms have recently begun after aerial yoga and after my hand was very cold on a snowy day. How can I know it's not ALS? I was really confident it's arthritis, but ultrasound didn't show it, therefore I'm so worried.