Cat limping on 2 legs down left hand side of her body?

I'm very close to my cat and about a week and a half ago I thought it was strange she was being distant and noticed she was limping. Only till today did I notice she was scratching at my door but when I let her in she was meowing hysterically at me. She's a old cat and she's about 12 years old I reckon. (don't know exact age) the vet said she was about 10 ages ago and my mum keeps saying she's about 10 but I can't quite see how that's possible. I thought it was just one leg but then when touched her front paw on the same side she started spitting at me and this is very unlike her as she's besotted with me. I think it's slightly odd how she has got incredibly worse over 1 day as yesterday she was sitting on my lap (not quite right) but she was purring slightly but we had company so she ran of like normal but then it wasn't that noticable. Please reply I'm very worried about her. Thanks x