The kind of muscle aches (myalgias) can be the starting symptom of a lot of illnesses. It CAN be the first sign of the plain old flu, but depending on what part of the country you're in it's probably a little early in the season for traditional influenza. Last year the Swine Flu started in November, but generally peak season for influenza is February or March.
And influenza can come on slowly. What is commonly called a "flu bug" is actually often another virus, which is why we're used to 2 -4 day illnesses. Many don't realize that true blue "brand name" influenza is a 10-14 day illness. The average healthy adult who gets the flu feels bad enough that even the tough will miss 2-3 days of work.
True influenza may develop symptoms over the first several days of illness. Importantly, TREATMENT CAN SHORTEN THE ILLNESS - BUT ONLY IF YOU START THE RIGHT MEDICINE WITHIN 48 HOURS of symptom onset.
If you have any rashes, or if you've done any travel, less common things may be going on.