If you Google "nitrofurantoin and blood glucose" you will see several articles which mention BG swings when taking this antibiotic.
How much carb though?
I hope you feel better soon. It doesn't seem to take much to upset stable BG. Lowering the carbs we eat seems to be the only thing we can control. Additional medication might not be the best idea because if the antibiotic is the cause of BG swings, you'd have a problem when you stop the antibiotic. It's the old balancing plates on sticks problem.