vaccinations are usually given as a sub cutaneous injection (under the skin in the tissue) sometimes these can stay under the skin to be absorbed but usually not over more than a couple of days. it's possible the lump is from something else. do the kittens play or fight? might be best to give your vets a ring, they won't do the injections any earlier but they will give you some advice as to how to deal with the lump to be on the safe side.