I have just re-purchased a perfume, but the fragrance seems different.

The impression of a fragrance may change from directly after opening to just before finishing the bottle, even if it’s the same fragrance.
This is because as you use your perfume, the alcohol is volatilised little by little, and is gradually used up. In addition, the fragrance in an almost-used-up bottle will have aged, so the colour and fragrance will be slightly richer.

If you want to enjoy the original fragrance as long as possible, replace the cap firmly after use, and store the bottle at room temperature away from direct sunlight in a location with a constant temperature.
The way you sense a fragrance will also change depending on the season and climate, and where you are at in your body and mind.