Many people can take statins with no problems, but others find they can have bad side-effects - including muscle and joint pain. Why not Google "statin side-effects" and have a good read. Also Google "spacedoc.net" and have a read of him. See what you think and then go and talk it through with your GP. Some statins - eg Simvastatin - seem more liable to have side-effects than others.
I don't recall muscle pains being a side-effect of Metformin. Does it say anything on the leaflet in the box? Google that as well if you're unsure.
As for the extreme thirst - are you peeing a lot as well? What are you blood glucose levels like? Extreme thrist and peeing a lot are the "usual" symptoms of diabetes.
Otherwise, all I can suggest is that you go talk to your GP.
Viv 8)