I have a new hard painful lump where my xyphoid process is. i am thin so you can see the lump under my skin. it feels like bone and is tender. why?

It may be: Your xyphoid process! If it is where your xyphoid is, and it is hard and tender, that is probably what it is. In my 20 yrs as a Dr, I have seen a handful of people in my office with the exact same history. 100% of them had a tender! pronounced (large) xyphoid. They were all thin, is it made their xyphoids very prominent. But I would see your Dr to be sure.