Why does a bike ride help relieve pain?

Okay, so every time the lower half of my body is in pain, whether it be sore legs from a long day at work to menstrual cramps, a bike ride (average of 45 minutes) relieves that pain. Why is that? It especially confuses me when I'm sore because of hours of activity in the first place (I work at McDonalds... ya... a lot of moving around). So why does a bike ride relieve pain so well?