Why isn't my insulin working???!?!?!?!?!?

how are you feeling in other ways, could you be fighting off an infection?

Hmm.
If it were me there are 4 things I'd consider
1) maybe over xmas it was the less exercisey, more foody regime that made it go up, and only this week is it still up because an infection is there in the background... but if you are feeling well and continue to feel well, that doesn't seem all that likely as you say.
2) maybe it isn't the pen, but a bad batch of short acting insulin. Perhaps change to a whole new packet with a different batch number, and see what happens. You aren't doing odd things with it like leaving it next to radiators, putting it in the freezer, or anything like that, I presume?
3) maybe it's a bad batch of long acting insulin. Are you waking up with the same level as you go to sleep with? Eg even if it's high, is it consistently high (17 at night, 17 on waking)? If so, the background's probably fine. But if it's rising in the night, it could be something wrong with the background insulin, or, see my next point...
4) maybe it's just your body changing its needs. That happens. Could be your liver's decided to up its output of background sugar, and so you need a bigger overall dose of background insulin.

Might be worth playing around with these ideas...?

As well as the points Snodger has raised it could be you have a problem with your injection sites..... do you alternate between sites?

Often when we are ill it can take a day or two for the symptoms to hit and this can often explain unusually high bg readings before this time, hope you get them down soon!

Low grade infection with very few if any symptoms can bubble under for quiet some time and not come to anything and really effect BG control.
Try all the above first,it may be just one of those things and you may just have to nurse it along as you are doing with correction doses and be careful with diet for a bit. It has happened to me over the years hopefully it will settle and you will think what was all that about :***:
Have a word with your DSN if you really start to struggle..Hope it works out for you soon