Do you mean in her throat area? Does she eat ok? The first thing that comes to mind may be a small goiter- which is an iodine deficiency- but if you feed kelp or anything like that, that would be doubtful.. You could paint her under tail area with iodine (On the skin under the tail its self, not the genital area). If it absorbs in, keep painting daily until it doesn't absorb.
Can you take a photo of the lump? Another possibility is hormones- the lymph nodes and everything may have birth hormones that will subside in a few days. As long as she is eating, gaining weight, active and pooping and peeing, she is probably fine.