My boy trembles too, like 2 secs or so. Very much like a chill down his spine or like when one tasted something sour. I didn't know what caused it til I was changing his diaper one day and he peed and I saw him doing that tremble again! So I believe it's just due to peeing. We adults sometimes have that tremble too after we pee isn't it? :-)
Not sure if your baby is doing the same too?