HI XXXXXXXXX,
I would check inside the mouth. It's possible through play that one bit a lip or their own tongue. If you leave any chew toys for them then it could have happened that way. Look for any type of ulcer or abscess in the mouth also.
If it does not happen again I would not get too concerned since you have checked them out.
dog training consultant
24 years training & behavior,pet first aide, Therapy dog eval., PA Animal Response Team