What's wrong with my doggy?

Two days ago she started acting funny & we wondered about everything from an impending earthquake to a wild animal in the area, later that evening she had an unusual disinterest in her dinner. She eventually ate a small one & seemed to rally overnight getting her energy back. She was excited to go for a ride that morning, then in the afternoon she became lethargic & kind of sad. She is normally a hyper & I mean hyper ball of energy. It's actually like she aged 12 years overnight, she's moving around very slowly, barely eating at all, & doing this swallowing thing a lot like she is keeping a burp in. Her coat seems suddenly greasy & she's just drinking more than usual, unless I'm just watching closer. She just lays there not wanting to do anything. She'll be 3 in a couple of months & I'm really scared this is something serious. It's a drastic change in her over 2 days.