Idk what to think ?

Sooo a couple of days ago one night my son (2 years) kept crying and crying all night. It was Sooo random it would be 2 am and kept crying and crying. Well let just say we didn’t sleep that night that happened only once that night. Fast forward to today it’s my daughters (3 years old) turn she was fine earlier. She went to sleep and she keeps waking up crying and crying soo random none of them are sick. But it’s a little scary not knowing why? I even called my husband since his at work. I mean they both cry so much idk what could be it’s not like them to do this. They are both good sleepers