It's not standard English (standard would be "I was sitting") but it is a common dialect or colloquial form in the UK. I think I might occasionally use it myself. "I was stood" is similar.
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GPY"I was stood" is similar.I've even heard "I was stood standing", but that was on a British sitcom, so it might have been said just for comic effect.
KJinCali79 It is sometimes called Multicultural (Inner)-London English and has influences from the Caribbean as well as traces from historical Cockney.Are you quite certain about that? As far as I am aware this is a dialect form used in various regions of England and has nothing specifically to do with London or multi-culturalism.