possible reason for sudden death of cat?

Question
I was taking care of my sister's cat this weekend.  I arrived at her house and her cat was perfectly fine, purring and rubbing her head against me.  When I went to leave, she howled and I went to comfort her, thinking she was just lonely, however I found her panting heavily.  Shortly after, she vomited (food only, no blood) and had a spontaneous bowel movement (stool looked normal).  I put her in a box and took her to my car and she raised her head with control however within the next 10 minutes her breathing became more labored and she seemed to lack bodily control (her head began twisting backwards).  When I got to the vet's office, she had red foam coming from her nose and within about 5 minutes, she passed away.  I haven't been able to speak to a vet about this and I am incredibly concerned because firstly, I am pregnant and secondly, I have two cats myself and I don't want to pass anything along to them.  I washed my hands thoroughly before I touched my cats, but I'm not sure if that's enough.  Do these symptoms sound like something you recognize?  Do you know what could have caused her death?

Answer
First of all, you are more safe than not safe from anything this cat might have had. Secondly, the other cats are probably okay too.
Now I don't know where you live, or how old she was, but what you described sounds just like the sudden-death syndrome cats exhibit with heartworms.

Cats do get heartworms in almost every state in the US and in other parts of the world also. It is hard to find in cats and unfortunately this is sometimes the only symptom.

When you say she had red foam coming from her mouth this was frothing from the lungs, which is another sign it could have been heartworms.

Cats certainly can and do die of heart attacks but it so very rare.
The panting, the howling, all point to some sort of heart and lung involvement.

Cats do get bit by black widows and can have a reaction like this, but again that is very rare.
Cats are rarely poisoned because they are too picky about what they eat and will not eat anything that smells slightly off. So usually whatever happens like this is a natural occurrence.

The only way to know of course was to do a necropsy on the cat and look at the heart as well as inside for heartworms or muscle damage to the heart walls. What did the vet say when you got her into the office? Did he have some idea what could have happened?

I am really sorry this happened while you were on babysitting duty- I am sure you felt terrible about your sister's cat.
You did all you could do.