Elizabeth - I agree with everyone else, best get to the doctor and get this checked out. If it was a one off I'd think maybe it was a virus you had picked up, but if it's happening frequently that would be a concern.
Kaz - My way of doing it is the midweek mileage reduces by 10% each of the first two weeks, but I cut my LSR down to 14 miles in week one and then down to 10 miles in week two (that's down from 22 miles as my final big LSR).
I ran my best marathon after doing no running in week one of the taper after injuring my toe. Part of me still wonders if that was actually a good thing as it forced me to rest that week.