Hello, i feel a couple of small (2 mm) tender moving lump over central area of my sternum just under the skin: are those lymph nodes or fat? thanks!

That's small: 2 mm is about half the size of the diameter of a headphone jack on a cell phone or laptop. Lymph nodes are not that small, and fat globules that small probably would feel like normal skin. A tiny 2mm lump in the skin covering the sternum might be a comedone, because comedones do occur in that area, and they would be about that size. Of course, it takes an exam to help decide what the tiny lump is.