Actually the sentence isn't complete you should say "walked in the garden" or "walked in the stree" or any onoteher place and this means "pass in the garden" or "march in the garden" i hope that i could help you:)

In that case, it means "You were trouble when you entered." Although, that sentence is awkward. I would say something like: "You were trouble from the moment you walked in." or "You were trouble from the day you walked in to my life." Trouble here means source of trouble.

'you were trouble when you walked in.'

Not likely. It usually means to enter. Do you have the entire sentence?