My female cat was recently in heat and around a lot of outdoor cats. Suddenly she started drooling from the mouth. What can this mean?

She might have a cold in that case it would be common Call your vet

I suggest taking her to the vet immediately. She could have rabies. Being outdoors she is around a lot of different animals who could have come in contact with a rabid animal.

Cat's don't drool as a rule like dogs.

Unless Your cat is foaming at the mouth It is not rabies
If your cat is drooling and it is like like water,
It could be the following

Ptyalism is the term used to describe excessive drooling. Animals might have increased salivation associated with certain stimuli, for example the smell of food, but when its onset is sudden and it persists, it is usually associated with illness or injury.

There are a whole host of possible causes why your cat might be drooling excessively. Here are just a few: Gingivitis (inflammation of the gums); Dental disease; Tongue injury; Mouth ulcer; Oral or esophageal tumor; Nausea and others.

You need to take your cat to the vet for a thorough examination, and normally this will include just the oral cavity unless this fails to find the solution. In the meantime, look out for vomiting, loss of appetite or difficulty breathing. Any of these symptoms might suggest there are complications that require immediate attention.