Does the molecular structure change over time?

Something else that I’ve been wondering about for a few months now is fragrance oils.
I get them in a really small bottle and love the one in particular. It has a little roller and I put it on pulse points and smell good all day. It’s fairly viscous.
But when the bottle is about 80% used up, the color changes slightly, it loses most of its viscosity, and the actual fragrance changes.
Is this because all the time I’ve been putting it on some oil from my skin has been going back into the bottle and altering the molecular structure? Is it possibly exposure to air? Dead skin cells?
What happened? This occurs every time.