Thanks Dave, now I know I'm not doing anything wrong (Yes you're right, my body seemed to sink evenly). One more thing, that sinking position started even when I tried to lift my hand to start doing the zen so I guess it is still normal right? So, I guess even in whole stroke that slight sinking when we lift our hand is normal (I hope I'm right).
The other day I tried swimming by using swim cap (Nike silicone cap). I don't know what went wrong but when I swam my head felt very heavy seemed unruly so I just took it off and everything went normal again.