Slight change in lesion size post treatment can be incidental and a few millimeters more can be just because the CT measurements are operator dependent (I suppose the lesions are in the same region i.e. Level VI, VII). Pain in jaw can be because of some unrelated condition. The throat infection
which you are stating can also be the reason for slight increase in size even though usually this does not affect level Vi, VII lymph
nodes. Just take a short course of antibiotics and analgesics. You can go for a CT guided FNAC of that lesion to rule out any residual disease. A PET CT
can offer some help is case a recurrence is being suspected. Regards.