Hi Gemma. Thanks for your question.
I am a board certified neurologist in the U.S. I will be happy to help.
If you are developing numbness on the left side with the constant headaches, and if the numbness symptoms have never been evaluated with an imaging study, then I would definitely advise talking to a doctor, preferably a neurologist in the very near future about having a Brain MRI done if possible. It will be important to rule out any structural problem with the brain, including inflammation, scarring, tumors, strokes, etc.
It is very possible that the numbness may be due to what is called a complex or complicated migraine, which is not usually a dangerous condition and will not generally cause permanent neurological symptoms. But until a brain scan is performed, and a proper history and examination is done, this cannot be ruled out entirely. Your next step should be to see a neurologist if possible in a prompt fashion, and this doctor should be able to guide you in the right direction including probably getting a brain scan. Best wishes.