Pain acheyness and trembly feeling in legs. some similar aches in arms but not as bad. been eating very low fat diet. related or what else?

Leg pains: Probably not. Why are you on a very low fat diet? You should see a good neurologist to get a good and thorough history and PE and if necessary some tests.

Hard to say:: Generally a "low fat" diet doesn't cause muscle pain or trembly feeling in the legs. If there was a connection to your diet perhaps your low fat diet is also eliminating health fats which the body needs? The main fat that you should particularly avoid are the partially hydrogenated(partially saturated) Trans fats. Trans fats are not a natural fat. They are found in processed foods.

Not clear: Could be many reasons. Are you dehydrated & not taking getting plenty of fluids? There's no need to eat a very low fat diet, especially since "healthy fats" like olive oil and fats in Avocados, are very beneficial and Your Body Needs Healthy Fats. Talk with your clinician to troubleshoot the problem and help you with a balanced Eating Plan.