Sri,
What country do you live in? I expect the form of this letter may change, based on the culture where you live. In the US, you would simply never say "your goodselves" to your boss. (By the way, that is plural, but you are writing to one person - so if this is really an expected form of address, it should be "your goodself" - but I repeat this is something I have never heard before.)
Also, very few bosses here will grant you entire month off for the flu - especially when your doctor said "a few days." That does not equal a month. Most US companies would require a doctor's note if you are required you to be out for any absense that long. Again, I don't know how it works where you are.
Do not say it's in continuation with your materinity leave. This is a new illness.
In any case, here are the grammatical corrections.
Last week, I became ill with influenza. My doctor has advised me to rest. I therefore am requestiong that you grant me medical leave for one month, from 1 December 2006 to 31 December 2006.
Thank you for your consideration of this request,
Sincerely,
Sri