Are you sure you can speak English?

Today, my boss had guests from a foreign country and he looked for somebody in our office who can attend to them to a railway station. He said jokingly (cos, permanent staff must speak English usually) " is there somebody who can speak English and attend them?" nobody replied. Every body seemed to be busy, and I know my colleagues don't like him so much. At that time, I wasn't busy, so I said to him "It doesn't include my contract, but if you want, I'll do it." He said " Are you sure you can speak English?"
Excuse me? why I have to say that voluntary, if I think I can't speak English?.....Am I fool???Obviously, he looked down me, because I'm hired as a support staff. He should know every body have secret abilities:)
I took his guests to the station by a taxi, helped them to buy tickets, took them to the ticket barrier and said good by. I don't know what the guests think about my spoken English. Actually, I haven't spoken English long time, so I couldn't speak English more than I thought. Oh, it's a excuse...regret always comes after, that's true,that's ture.