Reverse and high light on same time.

Just replaced my belt at 11,000mi. on my 2012 550. Ran great for about 100mi. But now my High and reverse light comes on at the same time and the engine wants to cut off on me. Reverse light goes off and it runs fine then it comes on again and it starts to miss again. Can’t figure this one out. Any one know what it could be.
Thank, Bob