After such a short time, the taste of your Macallan - as long as it's properly sealed - should not change so incredibly quickly. Perhaps it's your palate? Actually, the first time I had Macallan 18 yo, it was so amazing, I bought a bottle that week. The second time I had it, it tasted so differently - it was much better the first time at the restaurant. I soon came to realize it was my palate - definitely not The Macallan. Try complementing your scotch with different foods and you'll see how the taste can change. Let us know how it goes.