What Causes A Soft Lump On The Lower Back?

I've noticed a medium sized lump to the left of my spine in my lower back, went to put a box of books down the other day (they were kinda heavy) and felt this back pain, now it is killing me (lower back) Should it be anything to worry about, other than this i am fine and healthly. The lump feels soft and i've had something like this before and went to a Urgent care and they told me it was nothing to worry about.