Why is my dog throwing up?

There are many possibilities. You'd want to observe other conditions, not cover up symptoms by giving some medication that could worsen the issues.
Sometimes it's the body's natural way to get rid of something they shouldn't have eaten. However, sometimes vomiting can be a sign of a serious condition, from head trauma or toxin exposure to pancreatic cancer or gastrointestinal obstruction.

From PetMD.COM: What to Do When Your Dog Throws Up

There are times when a vomiting dog requires immediate treatment. If your dog exhibits any of the following symptoms, call a veterinarian. Frequent vomiting – dogs who vomit frequently can quickly become debilitated. This is especially true for puppies, elderly dogs, or individuals who have other health problems. Projectile vomiting – potentially a sign of an obstructed gastrointestinal tract Lethargy and depression – indications that the dog’s whole body is being adversely affected Severe diarrhea – the combination of severe vomiting and diarrhea can quickly result in dehydration Decreased urination – decreased urine production is seen with dehydration Abdominal pain and/or enlargement – these symptoms are generally seen with the more serious causes of vomiting in dogs Repeated attempts at vomiting but nothing is produced – this is a classic symptom of gastric dilatation and volvulus (GDV or bloat), a potentially life-threatening condition. The presence of red blood or material that looks like coffee grounds in the vomit – fresh blood appears red while partially digested blood resembles coffee grounds. Some causes of gastrointestinal bleeding in dogs are true emergencies. The vomit is bright green– some types of rodenticides (poisons used to kill mice and rats) are died a bright green color. These poisons can also kill dogs.

On the other hand, if your dog has only vomited once or twice and seems to feel pretty good, the following home treatment is a reasonable option: Take away all sources of food and water for six to eight hours. If your dog does not vomit during that time, offer a small amount of water. If your dog can hold that down, gradually reintroduce larger amounts of water. If after 12 hours of being allowed to drink, your dog is still not vomiting, offer a small meal of boiled white meat chicken (no bones and no skin) mixed with white rice. If your dog can eat this without vomiting, increase the size and decrease the frequency of his meals over a day or two and then start mixing in his regular food.
This whole process should take around three days. If at any point your dog starts to vomit again, see your veterinarian.

Like humans, there are numerous reasons that cause dogs to vomit. You can look up online to see causes and possibly narrow down the cause yourself, or take your dog to the veterinarian if the frequency increases, or occurs with other symptoms like fever, rashes, hives, etc or has been happening over an extended period of time (three or more days). If you notice any blood in the vomit or in their feces, get help immediately. Most causes are from something they've eaten, or abruptly changing food, eating toxic food (chocolate, mustard seed, pesticides), or eating random gross thing outside (dead animals, random feces, urine in a puddle, etc).

While your dog is currently vomiting, be sure to give them access to plenty of clean water unless they're gorging. This will help make sure that they don't get dehydrated. I have also heard giving them plain boiled chicken with no skin, bones, or seasoning helps "reset" their stomach so to speak. This also helps with diarrhea!

If you're concerned, I would suggest calling your veterinarian office and asking them questions. Normally the staff are very informative and can help with at home remedies if you're in a tight spot and can't afford to bring them in. They can also help determine severity with the symptoms you've noticed. Which can determine whether or not they actually need to be brought it, or if they have a sensitive stomach.

Good Luck!

The first instinct should be to call your vet. The staff there can advise you as to what actions to take, they may advise you to come in or a medication that could help. Vomiting can be caused by a very large amount of issues in dogs and generally you should call your vet when it occurs. If your dog's isn't vomiting consistently you may be able to start a bland diet. This is where you cook rice and boil chicken then give your pet the mixture with mostly rice to help ease the stomach upset.