From your symptoms,it looks like a Bartholin's abscess or cyst. There are glands
in the vaginal lips called Bartholin’s glands
. These can become painful and swollen, and an abscess can form. This often causes blockage in the main duct from the gland, and a cyst develops. This is felt as a painless, hard swelling.In such cases, antibiotics may be helpful, or surgical treatment might be required.