Galaxy S5 phone from eBay, nearly 4 months.

In the last few weeks it keeps shutting down, randomly. 

This is also draining the battery very fast, so I am only getting 8 hours or so from it as opposed to 1.5 days, which I was getting previously.

I have ordered a replacement battery from mpj but not arrive yet and I hope it helps. I suspect that this is a refurbished Galaxy S5. Anyone any ideas how to confirm it? Thank you.