Why do I have a limited range of motion in my heel/calf area?

I've been an active person for the past 5 years, and today I went to the gym, only to discover that any movement that involves my calf down to my heel is rather painful. It's almost like having shin splints backward. I can't make lunges, and simple straight leg stretches are tough too. I didn't want to push too much, so I stopped and left the gym, grimacing at the pain while walking. Should I be worried? How do I know if it's just a pulled muscle or more than that?