I totally agree with Archna.
Barring some really unfortunate circumstance which may have befallen your employer since your offer letter, hardly any employer will even want to go through the whole recruitment process twice in a month - for the same position!
And yes, your post doesn't appear as if you were surprised at getting it. Like Archna said, you sound as if you were expecting it. And, now, the crux of your question is - can a termination letter be issued prior to an appointment letter? In essence, you are seeking information on the technicality and mechanics as opposed to being shell-shocked at being fired within 15 days!
And, just in case, you're really innocent, then my suggestion is to detail the truth as Archna requests you to.
Else, I think your new best friend should be your lawyer!
Kind regards,