Short 2 Wheat at 986.50

Results:
+3.50 and -1.25 or $112.50 (before commissions)

I put a sell stop order at 988.50 and I got filled at 986.50!!! On the 2nd position, I thought I moved my stop to b/e +1 (I didn't know I got filled at 986.50), but it was -1.25 and it all happened so quickly.