When I exercise I get weak & shaky. What is this & how can I stop it?

Many possibilities. Feeling weak and shaky during exercise can be due to a number of causes. Most commonly, this is due to low blood sugar. Caffeine before exercise can also cause these symptoms. Lastly (and more seriously) a run of tachycardia (rapid heart rate) can also cause these symptoms. Eating lightly before exercise and avoiding caffeine is easily done, but see your doctor if symptoms persist.
See your GP. First see your gp for evaluation and blood tests. If normal then see a neurologist.