T2 Diabetes BG levels when on antibiotics

I have had a really virulent chest and urinary infection over the last three weeks and been on Keflex and Doxycycline antibiotics which dont seem to have cleared things up. I am T2 and take 2 x 500 Metformin SR a day and whilst I have been taking these antibiotics, my BS seems to have risen from an average 6.6 fasting up to between 7.7 - 9.8 every morning. A friend tells me that this is quite normal (she is not diabetic herself but husband is) can others please confirm that I should not therefore worry about these higher readings and should they return to normal once I can rid myself of these infections? Any advice would be appreciated.