Started penicillin wed. Thurs night vaginal itching. Painful invisible lump right labia fri night. Subsided next morning. Now, just severe itching.?

Lots of things. May be unrelated. Antibiotics can lead to a vaginal yeast infection. Should not cause the bump. May be worthwhile to have a quick exam.
You may. Have a yeast infection from the antibiotics. If the lump at 700 or 400 it may be a bartholins abscess which can improve (if caught early) w/ antibiotics. Try a monistat suppository. Good luck.