I Recently Found A Decent Sized Lump On My Back

I recently found a decent sized lump on my back just at the base of my ribs - about 1 inch in diameter and about 3 inches from my spine. It is fairly hard but not painful unless quite a bit of pressure is put on it.

Some possibly relevant medical history: I have ankylosing spodilitis but am very active and exercise consistently. My allergies have been pretty bad lately as well. Definitely some post-nasal drip, especially at night.

I also have had a sharp pain on the front of my chest, near the bottom of my ribs. Comes out of nowhere and is fairly painful for a few seconds - then it disappears.

If you can shed any light on the lump that would ease my mind but I will definitely be in to see a physician next week.