Could be for a number of reasons, bad insulin; bad pen; bad injection site; illness; just todays quirk.......I probably could go on.........and on...........
Any chance your insulin pen may have been affected by the cold weather we're having at the mo? Change the insulin cartridge/disposable pen first. Ensure you're injection sites are ok, if in doubt try somewhere you haven't injected before - within reason!
illness, stress, other environmental affects all have an effect, usually people realise they're coming down with a cold because of unusual high numbers before cold symptoms appear.
But then again it could be just one of those things, but a high of 19 is a bit extreme.