UPDATE........

Must have been stung by a bee or swasp. The swelling was completely gone 14 days later - she was very itchy for a while and lost all the hair in her gullet and cheek area.

Teeth and glands were checked and were fine, hairs has now grown back fully.

Thanks

My horse developed a lump at the end of his nose about 2 years ago and he had a fine needle sample taken out of it by the vet, it turned out to be parasitic.  We had just bought him and given him a 5 day course of panacur which led to him passing a large amount of red worm, the vet thinks one of these may have burrowed into his skin and this led to the swelling,  It disappeared all by itself in about 2 weeks.  If the gland is still enlarged it may indicate that there has been an infection which is being dealt with by the lymphatic system i.e. it may be an enlarged lymph node, a bit like when we have a throat infection, ear infection or tooth infection and our "glands" come up - these are actually lymph nodes and in humans a reactive node should settle in about 6 weeks, not sure for horses but if it doesn't go away or start to reduce soon I'd seek the vet's opinion.