Here's some info off the net. I would at least call your vet in the morning and mention that both your cats are now reacting.


Feline Heartworm Disease

Heartworms are a parasite of dogs and wolves. They don't really belong in cats, but when a heartworm-carrying mosquito bites a cat, the baby heartworms do the best they can. Because cats are a crummy place for heartworms to live, most of the baby heartworms will die and those that survive won't be able to reproduce. However, those little baby heartworms will do the best they can and some may survive. Unfortunately, in cats even one or two adult heartworms can cause a lot of trouble. The cat's immune system often reacts to heartworms in strange ways, so symptoms of the disease are unpredictable and seemingly illogical.

Symptoms
Usually there aren't any symptoms at all. When we see symptoms, they usually resemble other more common problems.

wheezing, choking
vomiting and gagging
fainting spells
hindquarter paralysis



I also found this info on Gagging in Cats and Dogs

Gagging

There is a place at the back of your pet's throat called the pharynx that acts like a switching station. It is responsible for sending food to the stomach and air to the lungs. When confronted by something unexpected, such as a trickle of postnasal drip, the pharynx doesn't know where to send it. It responds by gagging.

For dogs and cats, an occasional gag is just business as usual, says J. M. Tibbs, D.V.M., a veterinarian in private practice in District Heights, Maryland. Older dogs are especially prone to it because they produce a lot of mucus and their pharynges get more sensitive over time. When your pet is gagging frequently, however, there may be an infection or another problem that is irritating the pharynx.

Dogs with kennel cough, for example, will often gag. "You will hear 'cough, cough, cough -- ack,'" says Patricia Shema, V.M.D., a veterinarian in private practice in Glenn Dale, Maryland. "The ack is the gag. It sounds awful, but it is the cough that you have to worry about," she adds.

Anything that produces a lot of mucus, from viral infections to allergies, will often cause dogs to gag, says Dr. Shema. So can a more serious problem, like a weak esophagus or cancer of the throat.

Some pets have anatomical peculiarities that make them more prone to gagging, says Dr. Tibbs. English bulldogs and pugs, for example, often have extra tissue at the back of their throats that can be very irritating to them. In addition, cats sometimes develop polyps in their ear canals. If a polyp dangles into the pharynx, it can cause gagging.

In general, cats don't gag as often as dogs. The one exception is when an upcoming hair ball hits the throat. Cats will gag once or twice, then throw it up. Tonsillitis can also cause problems. "Cats have six tonsils, so imagine how it would feel if they were infected," says Margie Scherk, D.V.M., a veterinarian in private practice in Vancouver, British Columbia.

Pets will gag when their collars are too tight, says Dr. Shema. It might not happen when your pet is lying around, but you are sure to notice it during walks when the leash pulls the collar tighter against his throat.

See Your Vet If...


Your pet gags only when he is on a leash

Your dog is coughing as well as gagging

Your cat is coughing up hair balls

Your dog or cat has a runny nose and scratches frequently

Your pet can't open his mouth or is having trouble opening it

He can't close his mouth

He won't eat or has difficulty chewing or swallowing

His tongue, lips, or muzzle are swollen

There is a foreign object stuck in his mouth

His gums are red and swollen, or there is bleeding

Your pet is drooling or panting excessively

His tongue or gums are blue or pale

Your pet has ulcers on his tongue

He is gagging frequently

There is a lump anywhere on his face

He is pawing frequently at his mouth or face

There is a discharge from his mouth or nose that lasts two days or longer

His breath is consistently bad

Your pet's nose is dry, crusty, or bleeding

His mouth is foaming, or he's grinding his teeth

There is dried saliva around the mouth

Next Steps: