What Causes Body Ache And Muscle Soreness?

I went bike riding yesterday (20 miles one way of basically flat surface with a few hills), I brought snacks, water etc...) I got to my destination, visited a friend, ate dinner. Three hours after I had arrived, I headed back home. My thighs were in intese pain. Not necessarily muscle soreness... I couldn't ride. Walking helped a little and it really got worse when I stopped moving. Stretching did not help. It hurt so bad, my friend had to come pick me up with my bike and take me home. My thighs hurt so bad, I was crying. My legs felt heavy. I got home and took three advil and put Ice on my thighs. This helped. I could barely walk to the bathroom. Last night the pain eventuall lifted and my muscels are barely sore from the ride at all. I am almost back to normal today and last night I was crying. What is it? This has happened to me after a long hike and another bike ride in the past but it seems to be getting worse...