It's not unusual - you've just had your mortality gene kick in. It happens when you realise you're doing something in life that could kill you. It simply means that you now realise that what you're doing is "risky". That's good because it means you'll be careful and consciously think about what you're doing. The real "risky" activities in life are those where you don't realise how close to the edge you're operating and it's very easy to stray across the line.