If she's actually drooling or foaming at the mouth, she could have some sort of medical issue, ranging from overgrown teeth to an infection. Your best bet would be a proper exotics veterinarian to diagnose and treat her.
From experience, a respiratory infection's running nose can look like "foam" or drool in the mouth area given they are so tiny!
As for other causes, there may be a few:
- Depending on temperature, (if it is getting colder), she could be growing thicker/longer fur and eating more to prepare for winter. This would not explain mouth issues, but it would explain the change in weight/activities.
- Depending on her age, she could be getting older and slowing down. Older mice tend to be less active and may gain weight due to abundant food.
- Feeder mice are generally not bred with long-term health in mind. They tend to live shorter lives on average due to poor husbandry practices (cancer, inbreeding, genetic disorders).
- Fancy mice have a lifespan of 1-2 years on average.
As you mentioned this is your first mouse, does she have any cagemates?
Mice are social creatures and the companionship of other mice is highly recommended.
If she does have cagemates:
- If she recently lost one, she could be depressed. Depressed mice usually show lower activity and are more likely to fall ill.
- Check to make sure one is not male. Male mice have quite the distinctive cage odor, and are usually obviously equipped as adults. If you do find any males, she is likely pregnant and should be separated from them immediately. There is plenty of advice out there on how to best handle pregnant mice.
- Alternately, try isolating her to see if she is being bullied. If she perks back up, you can try trimming the offender's whiskers (only once, and no more than halfway!) and see if the behavior improves. Otherwise, she may be suited for a solo life.
If she does not have cagemates:
- How often are you handling/socializing with her?
- Has there recently been a significant change to your schedule?
- Without other mice, you are her socialization. Any change in these can negatively effect a solo mouse.
- If you try to get her a cagemate, make sure it is the same sex, and to introduce slowly and carefully. Not all mice will get along right away, and sometimes things just won't work due to each mouse's personality. In that event, make sure you have appropriate housing/the ability to return if things don't work out.
That said, not every mouse is suited for cagemates. I had one that was chill for a few months, murdered her sister, and started fights when I tried any reintroductions. She also was the (human) friendliest mouse I've ever owned. She lived to be 1yr 11months, so quite healthy by all accounts.
Also! Do not be too hard on yourself if she doesn't make it.
- Mice are prey animals and do their best to hide any illness for as long as possible. She likely has been sick for some time by now.
- Exotic veterinary care is expensive for many, and often difficult to justify for cheap/old mice.
- Even with a proper vet, proper identification, and medication, treatment/recovery is never 100% guaranteed.
That said, if you have the resources, mice -especially lab/feeder varieties- are some of the most studied animals in academia, and any qualified exotics vet will have a good understanding of your little friend's kind.
Hope you both share lots of love!
Edit: Forgot a case!