My bitch is normally dead on time every six months. However she was ill recently and she had to have antibiotics for dog flu and she is now three weeks late is this normal??

Only a guess, but if she's late I would have thought the fact that she's been ill is more likely to have had an effect than the antibiotics.

Ok how long would an illness push back a season??

Depends on many factors. She may miss one entirely. Illness can do allsorts to the bodies natural flow.

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SOMETIMES YOUR KNIGHT IN SHINING ARMOUR IS MERELY AN IDIOT WRAPPED IN TINFOIL