Chemically speaking, you may not be the same person you were last week. Any number of factors can trigger a change in body chemistry, which can alter the skin's chemical environment. That in turn can affect how your skin responds to your favorite perfume. Everybody's body chemistry is unique, so the triggering factors and their effects will vary from one person to the next.
For example, the hormones androgen or progesterone affect oil production in the skin. Some foods and medications -- not to mention pregnancy -- can raise or lower androgen and progesterone levels. Because oily skin carries a scent longer than dry skin, a change in diet -- or maternity status -- could mean more or less frequent applications.
Sweat can impact skin chemistry as well. Heat-related sweat deposits salt and other minerals on the skin. Stress-induced sweat carries fats that are broken down by bacteria. The process can also change the body's natural odor, and not for the better. In both cases, the fragrance compounds may react differently to their new chemical neighbors. You may want to save your usual perfume for a time and try another scent.