It can be allergic dermatitis
Thanks for writing your query.
After reading your post, it seems that you might be having allergic dermatitis.
The lump can be due to localized swelling, usually associated with it.
You should apply locally skin creams containing corticosteroids
for few days.
lotion may also help in reducing the itchy feeling.
Oral antihistamine like Fexofenadine
120 mg once a day in addition, will be helpful.
You should wear smooth textured clothing to avoid irritation.
Also avoid soaps with harsh chemicals and perfumes.
Avoid contact with the ivy vines in future.
If the symptoms persists, get yourself clinically examined and investigated by a Dermatologist
I hope this is helpful to you.
If you have any further query, I will be happy to help.