low memory window won't go

I just updated my macbook air to the latest update and the when it came back on there is a window saying "your computer is low on memory", when I press close it instantly pops up again. I have checked my memory and I still have 43GB out of 112GB left so this doesn't seem to be a problem. This low memory window won't go and just keeps popping back up instantly once cancelled, it won't let me move it round the screen and I can barely use my laptop at the minute as it is blocking part of the middle of the screen and stays in front when I open another application.

Please can someone help as I am currently unable to use my mac due to this.

Thank you.