How do I find who logged in on my facebook?

Hello,
hey there all, someone used my password to I-chat with a friend of mine on facebook, . I don't know how they got my password, but my friend forwarded me the conversation and they pretended to be me. How do I tell where this is coming from. Is there like an IP address I can find on it or will it give me the network, area, or address that it came from . This is important and I really need an answer, please help.