supplementing w/ formula- pooping at night?

MLF135

I just started supplementing BM with formula in his bedtime bottle because my supply was low in the evening.  Ever since we started that, he poops twice at night.  He wakes up around 4am to eat and we change him (usually poopy) then when he gets up for the day at 7:30 he has another poopy diaper. 

Before we started the formula he was rarely ever pooping at night.  Is this common? Could it be an intolerance to the formula? (Its not diarreha(sp))