Sorry - no advice - just empathy! Diagnosed 6 months ago so still learning loads. Like you I found that my out-of-bed readings went up with a cold, then dropped then stabilised about 4 days later. The information I found suggested that when we fight off colds we release hormones that mess with our sugar levels. I took that to mean that there wasn't anything I could do really but just had to ride out the swing in sugars during the early stages of the cold. Mine didn't go too high or too low - just swung around a bit!
Anyone else out there with more experience who can advise?