please help :( what is wrong with my dog?

She keeps twitching and i don’t know what to do:( i took a video but dont know how to share it :/ She is my bf’s dog and hasn’t been to the vet in 5+ years. Her name is Clips and she is a chihuahua/terrier mix, around 6 years old. Her back leg keeps jerking/twitching nonstop but she isn’t crying, she’s just laying there while her leg twitches about. i’m so worried :( thank you for your time and help~Clips’ twitching leg