It's probably not sciatica. Sciatica is a persistent but usually intermittent condition and the typical initial symptoms are spells of tingling or numbness in one leg.
Possibly you managed to outrage some small muscles that were getting used for stabilization, but the problem wasn't too noticeable until you tried to lower your legs slowly.
If you felt no residual sensitivity at all, I suppose it's possible you pinched a nerve briefly.
Despite the name of the exercise, don't actually hyperextend the spine when doing this move, just fully extend it.