Welcome to cycling. It is important to give yourself a break every few days. When you ride, you actually tear your muscles down a little. When you rest, your muscles build back up - plus add a little. It's like they're saying "Whoa, she might want to do this again, so we better get some recruits in here."

Be sure that you are not pushing big gears (e.g., peddling against a lot of resistance) because your knee ligaments & tendons need time to get strong too. Ligaments & tendons don't acclimate as quickly as muscles, so be gentle.

Most of all, have fun. You are wearing a helmet, right?