Hi Lucy!
Yes, it will be the infection causing it. I am insulin dependant and when I last had a cold/infection, my BGs went into double figures and stubbornly remained there no matter what i did. I had to double my insulin doses just to maintain 10s and 11s. This went on for about a week and then they came back to normal over a period of about 3 days.
You could try eating as few carbs as possible as you will be more insulin-resistant than usual, so any carb will have a bigger impact than usual, but I suspect your BG will still be fairly high - it's the body's defence mechanism. I'm afraid there is nothing you can do but ride it out.
Take care
Smidge