Please help me.

Hello to everybody! I’ve been using an iPhone 7 since February and I was very happy about the results, but lately my iPhone “acts” weird. For example a week ago I was on YouTube and when I turned my phone sideways to put the video full screen it didn’t respond... I tried it with other applications and it didn’t respond either. I restarted my phone and the problem was fixed. Today I was again on YouTube and the same problem occurred. What should I do? I can’t constantly restarting my phone! I have the warranty. Should I go to the nearest iStore and tell my problem there? Also sometimes the device lags for a brief time. Please help me. Thanks for your time.