In short, your clutch has had it and needs repairing urgently. This happened to my car not long ago - is there a burning smell as well sometimes? If not seen to, the clutch will eventually 'snap' (either the arm will break or all the teeth on the disk will break through) and changing gear will be a challenge!
My advice..... get it seen to and replaced - it costs around �225 if its a full replacement (arm + disc + labour). Clutches usually last around 70,000 miles but mine went at 55,000. I blame the driving style of the previous owner personally.....
Hope this helps :-)