My dogs head keeps jerking forward like she's going to throw up but she hasn't.

Does she have something stuck in her throat ?That would be my first guess.

What kind of dog is it? Is she retching, or just making the movements? One sign of bloat in a dog is unproductive retching.

Because nothing is coming up doesn't mean she isn't feeling nauseated.
Most times dogs will also make a licking motion when they are nauseated.
Consider other things as well.
How is her appetite? Her water intake?
Could she have gotten into anything? Has she had diarrhea?
Does she seem painful, uncomfortable, or swollen in her abdomen?
Try to open her mouth and take a peek inside to see if you see anything that shoudn't be there, toy, stick, anything out of the ordinary.
It is possible that she has something hung in her throat, it is also possible that something has thrown her digestive system out of whack.
Also does she take any medications? Some meds can upset the stomach.
The only real way to find out is to take her in to see the vet.