What Causes Spinal Prominence With Collapsed Disc And Back Pain?

I would like to know why my spine sticks out.It is only in the last year that this seems to have happened.I'm 5'6 120lbs,i eat healthily and run 3-4 times a week.Could it be that as i'm aging i need to put on more weight?I can more or less eat what i like without putting on weight.I have just been diagnosed with a collapsed disc in l5 region having suffered with lower back pain for more than 10yrs.Many thanks