Galaxy S6 :: Why Won't Phone Turn On And Randomly Turn Off

I bought my S6 about 3 weeks ago. I was using it normally, but then I left it and when I came back it wouldn't turn on unless I pressed the volume down key too. After that it kept turning off after 15-30 minutes I turned it on, everytime, and then it simply wouldn't turn on whatever i did, even if a pressed all 4 buttons for 10 seconds. Then I simply pressed the power button and it turned on. It behaved normally for that whole day.

I went to sleep with 45% battery and it died. Again it wouldn't turn on unmattering what i did, and after like 5 hours i pressed the volume down and power again and it turned on (after many tries), wtih 27% battery - and it was always off, but after like 2 minutes it died again, and only turned on, after many tries, when I presed vlm dwn, power and home together, giving me the restart screen with 7% charge. (not that dos/cmd-like, with hard reset and all, only a custom os and restart). it worked normally (draining battery fast), and then shut down again, even if i was charging it, though the whatsapp green led is flashing, so i think it's not completely off.