My cat tremors on one side + eye twitch

Sex: females Age: 2 years Neutered Duration: 1/2 weeks Species: cat Breed: domestic shorthair Body weight: unsure Location: UK Symptoms: tremors/twitching

So I got these two sister cats this summer. And I noticed that one of them, when they lay done on their left side, their leg tremors, or even their entire body seems to spasm basically it’s difficult to tell. Her eye on the same side also twitches sometimes. What could that be?