Why do my legs and lower back ache so bad when i just have a cold?

Cold?: A fever, headache, and other aches and pains are common with the flu but less so with colds. People who have colds usually have a stuffy nose and sore throat. That's less common when you have the flu. Flu symptoms often hit suddenly, which makes you weaker and weaker. A dry cough and fatigue can last 2 to 3 weeks. http://www.webmd.com/cold-and-flu/cold-guide/relief-from-cold-aches-pains#1

Muscle aches : Muscle aches and pains may indicate you have influenza or a more serious acute upper respiratory infection than you think. Any fever/chills? See urgent care md for full history pregnancy test if indicated and physical exam.

Interferons: One of the many responses to viruses is the production of interferons as part of an immune response. A side effect of interferons is aches. This is seen when interferons are injected as part of therapy for certain viral infections.