I have a small muscle mass under skin at my back near the hip. it's not looking like a lump or hard. only if i press the skin i can feel the mass.

Likely Benign: Most likely what you have noticed is something called a lipoma. These are usually harmless but it is impossible to tell you without actually examining you. If you notice it enlarging I would get it checked sooner than later. You should go for an exam every year to see a doctor anyway for a regular physical so I would just have them look at it the next time you are in the company of a physician.