Lily is an 11 week cocker.

Sometimes, when we put on or remove her lead, she becomes aggressive - snarling, trying to bite, twisting around so we can't get near her collar, etc. It tends to be worse when she is tired or over excited and when she comes in out of the garden. She is going out off the lead, but when we try to put it on when she is in the garden, it is bad. It also generally seems to be getting worse.

Any advice as to what we can do to stop this, please.