Am I in trouble when British people call me Trouble?

I'm going to try and give you a more comprehensive answer than all the other replies that simplify this as "it's a term of endearment". As with all English language - the context is importatn. Technically it isn't a term of endearment. In a literal sense it means 'this person is troublesome / they create trouble'. When someone is not present, it would not be positive to refer to them as "trouble" e.g. if in conversation with a friend you described a third person as "trouble" - this would be construed as describing them as 'problematic'. It is for this reason, that we use it 'ironically' or 'sarcastically', in person, to refer to someone as "trouble" (e.g. "Hello trouble" or "Look...here comes trouble!"). In person it is almost exclusively meant as a fun and bashful way of describing them: meaning you are exciting or adventurous or fun. It would usually be accompanied with a smile. You can of course still use it in a literal way if you wanted to directly say someone was problematic. But you would need to say it in a serious way.