Why is my iPhone 6 glitching?

Okay so I bought a brand new iPhone 6 last year when they first came out. Never had an issue till recently. I downloaded the new software roughly two months ago (iOS 9) and noticed the screen constantly freezing. I first thought it was a minor glitch and thought everyone with an iPhone might have the same issue. Not the case. I thought that maybe with the new update (iOS 9.1) it would fix the bugs. Since downloading that my phone became worse. Every few days it would glitch and start opening random apps or if I'm texting people start typing random letters or even delete what I've already wrote. It almost looks like a ghost is controlling my phone honestly. But I just dealt with it because it would only last for 30 minutes but since 4 days ago it's been completely unusable. I go on twitter and it retweets everything for me, I can't even text people. I can't even type in my passcode to unlock my phone because it either types in random numbers for me or deletes the numbers I try to type in, the only way I can get in is to use the finger print ID. So today I spent two hours on the phone with Apple support, first he troubleshooted that I should restore/restart my phone through iTunes because I originally downloaded it over Wifi and he said that by doing that and possibly not having a strong connection, it could leave out some data from the update. We did that and that didn't work. He then got word from his supervisor to do another restart/restore but this time set my phone as a brand new phone. After doing that it seemed my phone got only worse. He did a diagnostics on my phone and found nothing wrong from his end. He finally suggested I just go into the store for them to take a look. So I go in Monday and thats fine hopefully they can help but I'm so curious and frustrated because I have no idea what caused this or is this just one bad egg in Apple's product? Is it random and Apple's "fault" because of their faulty product or could it of been something I've done? What would cause this is what I'm wondering. My phone doesn't have any water damage and the only blemish on it if you will, is a small dent in the bottom right corner of the phone not even near the screen and that happened in January. I have no issues charging it or using the headphones. I don't download anything onto it that isn't from the app/itunes store. I haven't heard of anyone really hacking the physical iPhone, I know people can hack your Apple ID/Password but I know that's not the issue in this case.

Basically I'm at a loss and I would appreciate any insight. Thanks.