I got the exact same today from my Ce4, but I think I know why. It's happened before a couple of times. I had been using it exclusively at work for a couple of weeks and I think it had taken a few knocks so that the tip wasn't seating off squarely against the seal. The first clue that this has happened for me is that the juice turns very dark over a day or two (I'm assuming from the broken seal?) and the flavour changes drastically to almost burnt. The occasional pull on it fills the tank with vapour too.
Today it literally went from just ok to having a massive coughing fit and the most acrid taste. Changed it out for a new one with fresh bright juice and it was all good again! I get ce4's from eBay for very little and I normally get a good 3 weeks or so out of each one before it's been knocked about so much it's useless.