With a fifteen pound weight loss that you weren't working for and unexplained fatigue, you ought to see a doctor.
Unexplained weight loss and unexplained fatigue can both be symptoms of some very nasty diseases.
Call for an appointment on Monday.
i've done so the last time this happened, i lost 30 lbs in 3 weeks, i told my doctor and he didn't mind he was actually happy cuz apparently i'm "obease" he still wont care because i now weigh 168 and i SHOULD be at around 125-130 most he said. so he doesn't care so long as i lose weight, even when i explained to him that i haven't done anything to lose it, nothing at all.
I'd still make an appointment to see your doctor.