Unhappy [Q] got 0 on imei and phone perform reset

hi
need some help on my galaxy S4
model: SAMSUNG-SGH-I337
android version: 4.2.2

1. why i got 0 (zero) on my IMEI ? (I can make and receive calls)
2. why my phone perform reset (No predefined times) - I think when is in effort - But not sure about it
3. How do I know the phone is not fake ? (I bought it two days ago)

thanks in advance