Please note: I am not a vet and what I am about to say is not medical advice. It is advice based on personal experience.
That swelling is most likely based on an infection. A qualified and experienced vet should draw the fluid off with a syringe and send it for lab work. They will prescribe antibiotics and may need to change the antibiotics after the lab work comes back.
So take it to a vet, follow their orders, and it'll be ok. If you ignore it, the infection could become systemic. Here is a thread with a snake that has a systemic bacterial infection. It's not pretty:
Very Ill Elderly Snake
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