coworker possibly pregnant with her tubes tied!

so...this happened today. she kept getting sick at work whenever anyone would cook anything on break so jokingly we told her to take a pregnancy test and it came out positive. now she had her tubes tied 4years ago after the birth of her second child. what are the odds of this being a false positive? she's going to take another tomorrow morning so I'll keep everyone up to date but is it possible that this soon after having them done that they could reverse on their own?