small lump under chin- to wait or not to wait?

I noticed yesterday ( or maybe two days ago) that my male 2 year old cat has a lump under his chin. I rub his chin all the time so I am fairly certain that it is new as of the day I noticed it. It is the size of an pencil eraser, the ones that come attached to the pencils. Today, I looked closer at it and it seems to have a slight redness to it and it looks like he may have scratched it with his claws- there are two parallel lines that bled slightly in a few spots. The fur is normal.  I think the lump might be slightly smaller than it was yesterday. 

... so I am thinking it may just be a scratch that got irritated/infected? I don't know what else it could be.

My question is this- When do I call the vet? I am traveling for work the second half of this week... the cat sitter can take him if need be, but I would rather do it myself tomorrow if he definitely needs to be seen, rather than wait and let the sitter do it. 

If I were you, I would take him myself, so if it's an infection it can be treated during the second half of the week while you are away and you won't be worrying about it.  If you can't do that, just ask your cat sitter to check on it.  If he scratches it and breaks the skin, you could put triple antibiotic ointment on it to prevent skin infection.  It could be something as simple as a bug bite or as complex as a cyst or abscessed tooth.  Only a vet can tell you for sure.