There are many things that affect BG - food is the obvious one. But BG is also affected by time of day, month, year, illness, stress, drugs, alcohol, exercise, ...
Typically, when I am ill, my BG rises, I need extra insulin (either corrective bolus doses, increased basal or both) to keep my BG down. Unfortunately, for me, a high BG will magnify any aches, sniffles, coughs, ... so it definitely helps to keep my BG under control which means lots more testing.
Enough about me. The answer to your questions are that high BG can be caused by your cold and this is temporary - get rid of your cold and your BG will come down.
In the meantime, test, test, correct, test, correct, ... do not be afraid to increase your basal slightly whilst you are under the weather.