Fortunately the nearby vet clinic is open on Sundays so we were able to have our dog checked. There is a lump on the right side of her neck. The vet suctioned the fluid and the lump was gone. Hopefully it will not appear again.
@Alexandoy I wonder why the vet wouldn't have discussed the reasons with you. I guess it would be better to ask during your next visit so that you can see to it that you can avoid this from happening again as far as possible.
The vet's usually test the fluid to check what it it. It's helps in deciding the proper course of treatment. Hope your vet has prescribed antibiotics for your dog and it subsides fast.
Wishing your dog a speedy recovery.