Otto -Bobby has a swollen lower lip with a sore ( picture)

My little buddy Otto -Bobby went from being his playful loving self to a sad kitty with a hurt mouth,

2 days ago I noticed a small red bump on his lower lip then within 48 hours he's drooling, growls when I approach him to look at it and at one point stopped eating and drinking water and doesn't groom himself. thankfully he ate and had some water last night so I thought things were getting better but today he seems to just want to sleep , Does anyone know what this might be?

Please help...

No idea what it is. Email the picture to your vet and ask what it is. Since tomorow is Sunday you probably won't get a response until Monday. If you don't hear back from the vet, call and confirm that the email was received and ask if the vet has an idea what it might be and schedule an appointment if needed.

But if your cat isn't eating anything and the sore seems painful, I suggest taking your cat to the nearest vet ER right away for treatment. Cats who don't eat for more than a few days are at risk for fatty liver disease. The after hours voicemail message for your vet should have the phone number of who to call for emergencies. You can also look for the nearest vet ER here https://www.veccs.org/facility-directory/

Does your cat go outside? Do you have other pets in the house who may have caused the sore?

It could be what's called a Rodent Ulcer.  Those are often the location of them.  But if it came on suddenly like that, perhaps not.  Possibly some sort of bug bite?  Is it getting worse?