The girls are going for a checkup, but now I'm worried about my son's …

The girls are going for a checkup, but now I'm worried about my son's stool. He went five times yesterday. Two times was dark brown (but we ate prunes and that's okay), then two times as usual brownish-yellow, and in the evening very liquid but also brownish-yellow. Now he has a lot and mature. There is no mucus or foam. Could it be from the medication we were prescribed? The baby doesn't have a fever. He behaves as usual. Should we give him simethicone?