Poison.

If your dog suddenly falls down panting and shaking, vomiting and obviously suffering from severe stomach cramps, it may be a symptom of variety of disorders.  If it is not possible for you to take the dog immediately and you suspect that the cause is poisoning, then emergency treatment is required to induce vomiting.

 To  induce vomiting a common method is salt , one teaspoonful mixed with warm water; or mustard in the same proportion.     Forcing this mixture down the dog’s throat will cause him to vomit.

It will be helpful if you are able to establish the type of poison he has eaten before taking him to your Veterinarian for treatment as soon as it is possible for you to do so.