Been there, done that.
The first symptoms of too much nicotine are some combination of headache, dizziness and nausea. Oh, and some people get the hiccups. Many of us (like me, for instance) have made ourselves 'nic sick' by
vaping with too much nic or vaping more than usual while playing around with new equipment. There's no long term harm at all at that level (provided you stop vaping or otherwise exposing yourself to nicotine); you just feel rotten for the 10 - 20 minutes it takes your body to process the nicotine. If you didn't experience those symptoms, then the base probably wasn't on your skin long enough to absorb any significant amount of nicotine.
For storage, 'best practice' is to separate a large quantity of nic base into multiple small, coloured glass bottles and store it in a freezer (deep freeze, if possible). 'Small' might mean however much you go through in a few months. Then you can thaw it out a bottle at a time. Keep the bottle you're using in a cool, dark place. Some store it in the fridge, but that will still leave it pretty viscous, especially if it's VG, so most people just keep it in a cupboard - as long as it's not right next to the stove or anything else that will heat it up. You might also consider a safe or lock box if you have children.
Heat, light and oxygen exposure contribute to breaking down nicotine. The freezer reduces heat, coloured glass reduces light exposure and glass instead of plastic reduces oxygen exposure and also helps avoid picking up odours from other items in the freezer. Putting it in the freezer is very important if you want it too last a few years. The other two help, but are a bit less important. Nic base stored in clear glass in the bottom of the freezer isn't really going to encounter much light anyway.