This is tough since kids go through growth spurts and teeting and developmental spurts where they require more sleep. We have a 2 year old and I notice when he is teeting and sometimes fighting something (cold) his sleep patterns change.
I would say if it is very drastic and she seems lethargic then you should get her checked out...test her blood levels (vitamins and electolytes), iron (for anemia), glucose levels (blood sugar), and do a sputum culture to be sure she isn't harboring a new bacteria which may need treatment. Your docs may have other suggestions as to what to look for of course.
Is she getting good calories throughout the day? Does she have signs of allergies (I have hayfever in the fall and am feeling it big time...wanting to sleep more)?
I would use your gut on this one and if it makes you concerned at all, it can't hurt to have a check-up. It will ease your mind.
Warmly,
Jody