My dog is being treated for hook worms and last night I noticed a white worm on her rectum. Her stomach looks more swollen than it has and I don't know if I should call the vets emergency number to ask about it.
What do you think?

Is the worm rice like or does it look more like a piece of spaghetti?

Edit: Because I thought you were still on line and could answer that I didn't attempt to answer further without more details.
I suppose I was mistaken
But based on your question I will just give you some info.
Tapeworms are small rice like segments usually found on the rectum, where the pet sleeps, your lap, etc.
They are not an emergency situation, however they do need to be gotten rid of.
Tapeworms are the result of the animal ingesting a flea, so flea control is very important in preventing them.
There are dewormers that kill only the tapeworm and dewormers that work to kill multiple parasites.
Without the name of the dewormer you are giving I can't tell you which is the case.
Roundworms look much like a piece of spaghetti you would see them in the stool rather than on the rectum.
These are the only two intestinal parasites that you will actually see with your eyes, the others are microscopic.

Hookworms are dark also.. That should help identify what type of critter came out.