Oh my, I remember when I studied in London for nine months, and the went to the US for vacation! I had the same problem! If I remember it correctly, in the UK "Pissed off" is a swearword, but in the US it has more of a "being really angry"-meaning. I could be wrong though, since I´m not in any way an English-expert, and even worse, English is not my first language.
I also bumped into the problem with the word "pissed", which was very bad when I accidently used it at a US University. In UK it meant being really drunk.