Tiredness is quite common when starting to take metformin, some people compare it to a mild case of flu, not sure about the muscle ache as I never experienced that symptom mine related primarily with tiredness, nausea and dirrhea, the tiredness passed in a week the dirrhea never really went away.
After a year I changed to the slow release version of meformin, glucophage with this i have virtually no symptoms at all. if after a couple of weeks things dont improve it may be worth trying glucophage
If you find that the muscle pain becomes severe i would speak to your gp, metformin has been known in rare cases to cause lactic acedosis
Warning signs of lactic acidosis include fast and shallow breathing, diarrhea, severe muscle aches, cramping, unusual weakness or tiredness, or feeling cold.
As i say it is quite rare though