When you have lactic acidosis, you are very sick. The primary symptoms are weakness, nausea, vomiting, feeling cold, and yes, muscle pain. Muscle pain that is very, very strange is a hallmark.
LA is seen in 1 on over 33,000 people who take metformin; so you can see it's really rare. Risk factors for it include: being over 80, having kidney problems (since met is processed by the kidneys), and liver issues. Other risks include surgery or medical procedures that require dyes: these are again, processed by the kidneys.
Definitely contact your MD if you are worried. | 
neck pain 
