iphone 7 plus black screen glitch?

Hi, I just picked up my iphone 7 plus 128 gb last sunday and everything was good...until today. Well my phone was sitting on the table with 100% battery and I heard a text notification sound, so I reached for my phone and pressed the home button to check BUT the screen just stay black! I tried pressing the home & sleep buttons couple more times but didn't work (the home button still responsive/vibrated whenever i press). After 10-15 secs, the screen turned back on by itself and the phone felt really laggy/slow for the first few secs before got back to normal. So does anyone know what might cause this? is it a hardware or software problem? Just need some advice before making appointment with the genius bar.

PS: This is my second iphone 7 plus, first one was 32 gb version and experienced the same problem.