child bit at daycare

Daycare parents: my child (17 months) came home today with 2 distinct bite marks on him from another child. I was not told anything and didn't notice til I got home. When I asked a worker about it, she claims that IF he was bitten, he never cried to let them know, and she also suggested they were self inflicted. I know they aren't because he only has 2 bottom teeth and these are full marks. I haven't spoken to the director yet, but I'm curious... How should I handle this? Am I overreacting? I just wish they would have told me!