Fountain sodas are a crap shoot. Maybe the syrup bag is running low, maybe the carbonation source is about to fail, maybe someone switched around the diet coke and cherry coke lines and didn't switch the syrup:carbonated water ratios. Maybe the water line is too warm (which can also throw off ratios), or maybe a shifty manager adjusted the ratios to steal from the customers / company. Or maybe your ice just melted =)

Plus the whole smell == taste thing to account for. Maybe drinking from a straw obscured the smells you can get versus an opened can.