Not a doctor here, but am someone who has had sciatica. Not so much any more since I stopped all the heavy labouring in the yard. The angle you're bent into when you're riding might have something to do with it. Maybe changing the angle or lowering the seat might help a little. Depends what type of bike you're using. I can imagine a lot of bike riders would have had this problem from time to time. I'm sure this is not an uncommon occurrance. You're maybe triggering some type of physical or mechanical issue that is pre-existing in your spine. Have you ever had back problems before this? No curvatures or major injuries?

Anyway, welcome to the forums here. I hope you get some answers and can get back to riding pain free. The main thing is to figure out what is actually causing the pain. Investigations might be needed for that.

Added: Here's the link to the Spinal Disorders Forum where you could ask your questions there...

http://neurotalk.psychcentral.com/forumdisplay.php?f=22

and here's a link to an older post from someone else

http://neurotalk.psychcentral.com/showthread.php?t=8890

take care,
Lara