Wonder if the sit-ups I started 3 weeks ago started this? I've been riding this comfort/hybrid(really a comfort with 700C Wheels) bike for close to a year with the seat and handlebars set in the same position for nearly as long. In the last 3 weeks I've been doing sit-ups/crunches and push-ups. I would find it strange that doing these exercises would've contributed to the sciatica. I restarted riding after 4 weeks off the bike to let my hand heal from trigger finger release and carpal tunnel release, but I did step-ups(2 stair version, one step up then another step up then back down 1 step at a time) and giant step ups(taking the two steps in one step motion and back down to the floor).