Been to doctor about very small ball shaped moveable lump in side of neck after flu shot. she said it might be lymphnode.what else could it be? 21yo

Lump: Depending on the size and firmness it also could be a lipoma or cyst. I would monitor the lump, if it is increasing in size, firm, and immobile I would recommend followup. Lymph nodes less than 1 cm are generally benign but if persistent or increasing, it would be reasonable consider biopsy. A flu shot does stimulate an immune response so some inflammation to a lymph node is possible.