A bird that bobs its head up and down in a sort of pumping motion, beak open and then a purposeful delivery of partially digested food is regurgitating. This is something they would do to feed offspring or a mate. It's done by some birds to objects they are particularly fond of, especially if they're in a breeding season.

Regurgitating can also be a symptom of crop infection or other problems, but generally it's a threat because it can lead to malnourishment. If your bird is giving up too much of the food they should be digesting for their own nutrition, the result can be pretty serious.

http://www.avianweb.com/vomiting.html

Vomiting is more of a head ‘flicking' event. The bird will often seem uneasy, pacing or uncomfortable and although the head bobbing might be similar to the regurgitation action, it's usually more of a shaking and the end result is a very splattered, sticky substance that may or may not include food.

If possible, collect a sample of this substance to bring to the vet with you.

When there's a blood showing in the vomitus it may indicate esophageal or proventricular ulcers (http://www.multiscope.com/hotspot/pdd.htm) .

The vet will take a look into your bird's mouth for other symptoms and probably take a swab culture. A good exam will also check for any growths or tumors.

Vomiting is a more serious symptom and seeing a vet as soon as possible is important. There are far too many possible diseases to outline here, but as in any case of illness, getting it evaluated, diagnosed and treated right away is often the best outcome at the lowest cost.

The top causes of vomiting in domestic birds are (gram-negative) bacteria, something that may increase to a troublesome point after the bird is stressed somehow. Stress can involve changes in their environment, being frightened, having their sleep hours reduced or other changes in schedules or even a difference in food (which you indicate you have not done).

http://www.peteducation.com/article.cfm?cls=15&cat=1829&articleid=2418

The other possible causes are contamination of food or water by fecal matter. Be sure there are no perches above food/water bowls which could allow this to happen without you noticing until after the bird has ingested it.

Candida (also increased after a stressor) infection or Trichomonas are also causes that should be explored by your vet. When these are all ruled out, the search for a cause can get pretty complicated.

If you're dealing with regurgitation, as hard as it might be for you to do, remove the object of your bird's affection and begin to modify the bird's light and dark hours to help curb this behavior.

(References:

http://www.theveterinarian.com.au/clinicalreviewcve/article300.asp

http://www.peteducation.com/article.cfm?cls=15&cat=1829&articleid=2418