Canine Ladder of Aggression
Dogs are communicating with us all the time through their body language
A wagging tail can mean different things. It can signal agitation or upset as well as happiness, so it is important to look at your dog’s facial expression and their other body signals as well. Whilst some dogs are harder to ‘read’ than others, you can become the expert on your own dog and on what they are "saying".
The Dogs Trust have further reading on this topic.