After a good workout yesterday, today I find myself a bit sore. Muscles that haven't been adequately used for awhile are now reminding me of their existence. This phenomenon has a name: Delayed-onset Muscle Soreness, or DOMS.
DOMS can be anything from a mild ache to a pain acute enough to limit everyday activities. More severe pain usually occurs when a person first begins working out. After weight training for many years, I find that even after a 6 month layoff, my pain is mild. And I worked really hard! But I also train smart, and that is important.
No one really knows what causes DOMS. The theories range from a buildup of lactic acid in the tissues to microscopic muscle tears that cause inflammation. Whatever the cause, the pain is very real.
It's important to distinguish between DOMS and real injury. Muscle soreness from a workout can be classified as "good pain" in that it indicates muscle growth. "Bad pain" comes from an injury such as a pulled tendon or torn cartilage. Pay attention to where you are feeling pain to distinguish what you are feeling.
If you are in severe pain, it's okay to rest a bit before working out again. You might also try working those muscles that are not sore until the ache diminishes. Drink lots of fluids and rest as necessary to help the affected muscles recover.
And don't give up!