Riley, with Labs when they get a "fatty mass" it's a soft tissue mass, I dont know if this holds true with all breeds. Is there any possibility he was stung by something? I have had a 'kid' here and there get stung and it will swell and be hard until it goes away.
I would think if it were something on the more serious side it would not just appear over night but rather develop slowly. Give your vet a call and see what he/she says prior to the appointment Monday???
Dont get yourself stressed out until the vet has seen Riley. The majority of lumps and bumps are usually not life threatening. The suddeness of the lump, would make me think it may be a bite of some sort. Rocky got a big hard lump on his side one time. My heart sank when i saw it and i took him right in. It turned out to be a bug bite. If it does turn out to be something serious, you found it quickly , so it will be more easily treatable. Dont worry and keep us posted . I am sure he will be fine, he has a wonderful and caring mom !
If you are a dog and your owner suggests that you wear a sweater, suggest that he wear a tail.