I am now on half a dose of a muscle relaxers, twice a day. at night my legs cramp up and sometimes twitch. is that normal?

Dose may be too low: There are many different things that may cause this including muscle spasm, ridiculopathy or restless leg syndrome for starters. Since the treatment of these is very different work with your MD or Neurologist to adjust your medication and or consider other treatments,some are neuropathic,others include Physical Therapy,massage and stretching or my of asocial release. CHECK vitamin levels as well.