Welcome to the board, Nurisar!
My best advice: don't mess around with it.
I've been guilty of 'overchecking' in the past--if you keep checking the lump, it'll just get bigger from irritation. Does it seem to bother him? If not, just leave it alone till you can have it checked. If it seems to be painful, a cold compress might help.
It's a good thing you kept track of where the injection was given. This way you know the lump is likely from the vaccination and not a tumor. One of our dogs developed a lump between her shoulder blades more than 2 weeks after her vaccination, and we didn't connect the two events. The needle biopsy of the lump was inconclusive and the vet worried that it might be a mast cell tumor so we had it removed. Very traumatic, and the pathology came back as a benign vaccination lump. If we'd kept better track of where the injection had been given, it would have saved us a big bill and our dog an unnecessary surgery. Since then we, too, have been keeping very close tabs on vaccination sites.