There actually could be several different reasons why she's behaving this way, especially reasons why she might be excessively salivating or drooling:
1. Problems in the mouth such as an infected tooth, ulcers, abscesses, or foreign body. Even though you didn't see anything obvious when you took a peek in her mouth, it's still possible that this is the issue. Sometimes to perform a more thorough oral exam, sedation may be necessary to adequately examine the mouth.
2. Nausea for any reason. Systemic diseases such as kidney disease (she's a little on the young side for such an issue) or inflammatory bowel disease can cause cats to feel nauseous. Bloodwork might be suggested if this is suspected although inflammatory bowel disease is only diagnosed with a biopsy (understandably something that most owners are reluctant to have done).
3. Ingestion of any noxious substance that she might have groomed off of her body (such as flea/tick products) or anything that she might have licked. I wouldn't expect her to have other symptoms, though.
4. Early upper respiratory infection or Calici virus. This virus can attack the mucous membranes in the mouth and create ulcers on the hard palate and tongue which can be quite painful. Some of these cats will also develop an eye discharge and might run a fever. In most cases, they're also sneezing, though, which she's apparently not doing...not yet, anyway.
5. Spontaneous. In some cats, we can’t find an explanation and these cats will spontaneously stop drooling and we won’t have had any idea why they started in the first place.
Over the counter, human Pepcid AC (Famatodine) can be given if she might be a little nauseous; the dose would be ¼ of a 10 mg tablet twice a day. I wouldn't suggest any other otc medications, though, since cats can be quite sensitive to many of them which might cause more harm.
If she continues to behave as you've described, then a vet visit may be prudent but it doesn't sound as if this is an ER type of situation just yet.
I hope this helps. Deb