Lump on back of my dogs neck

hi last friday i took my dog to vets to find she was suffering from pyometra, they did emergency op to remove infected uterus (spayed) she got through the op well and is recovering well at home, however the morning after the op i noticed large soft lump on charlies neck i took her back to vets who told me it was likely to be reaction to the injection she had and that it would clear up. nearly a week on and its still there it is now quite hard and shaped like an egg it sticks out and is very noticeable she isnt in any pain it isnt weeping or red its just there, she is ok in herself and is eating well it doesnt seem to be bothering her at all. does anybody know what this could be? how long it should take to clear? and whether i should take her back to the vets or just see how it goes?

Dogs are different, this is very common, the duration depends on the individual.

What I do with lumps is this:

measure it, l x w and then you can determine if it is growing, shrinking or remaining the same.

If it is getting any bigger, take dog back to vet, if there is no change or it is shrinking, just wait.

ok i will measure it and keep an eye on it thank you for your advice! it isnt bothering her and ive been told its nothing to worry about but u just cant help it sometimes.... thanks again.