What is this lump on my pups head?

Hey vets, This is my boy Kyro he is 12 weeks! He goes to the vets Friday to get his stitches taken out so this will be addressed but until then I need some kind of answer. I noticed this bump and am freaking out. He did not have this at all today and out of nowhere he now has it. It’s not a hard lump and when I press on it it doesn’t hurt him and he’s still acting normal. Idk how he got it or what it could be.

Hello!  I wonder if this could be tissue fluid building up under the skin.  This is called a seroma and can happen when fluid flows to an area and then can't drain, particularly common under surgical wounds.  Other possibities could be an infection (abscess), a bleed (heamotoma) or inflammation.  It's definitely worth showing to your vet; perhaps you could call them to triage as to when would be appropriate.  A seroma would usually go down by itself with time but the others might be more of a concern.