Big changes, like starting a new school, moving house or the birth of a sibling, can trigger misbehaviour or feelings of anxiety or sadness, so its possible your child is upset or behaving badly because of something happening in their life. Often these feelings or behaviours are short-lived so it's worth giving things a bit of time before you contact your GP, but, if you haven't already, you should be having conversations with your child about what's going on.
While children can find it hard to put their feelings into words, from a young age they'll understand when you talk to them about their feelings and behaviours. Older children might feel more comfortable writing down their feelings to share with you.
Take a few minutes to read this NSPCC guidance on how to have difficult conversations with your children.