Apart from being incredibly flat I would say that it and you will be absolutely fine, especially as you only drank a sip. Dr Pepper is essentially just a mixture of carbonated water, caffeine, sugar and a bunch of preservatives... it is not exactly the ideal breeding ground for bacteria. They say to keep it refridgerated and consume within 3 days of opening just so that it keeps its freshness as it is pretty gross after this time as you probably found out after taking a swig? Don't worry, no one has ever died after drinking week old Dr Pepper. :P