Why is my phone freezing and restarting if I only got it 3 weeks ago?

I got an IPhone XR for my birthday 3 weeks ago, and about 2 weeks ago it started acting up. It would freeze and restart the whole phone and black out at random times. It's been happening a lot more recently, about 2 times a week my phone will act up. I don't know if the phone is just defective or what.

Should I get the iPhone 6S Plus?

I have the iPhone 6s, yet it's only 16 GB. Some stores have the 6S plus option for 200 for a limited time only. I get paid Friday, and I don't have much plans towards buying anything else other than the phone and paying some minor bills. (Phone, cable, etc). Should I splurge for myself a little and get the phone? And would you say this deal is good as well? Really would like to try the phone out.

What to do to fix this problem with my iPhone and iMessage?

So everytime I scroll through my attachments and pictures with people on iMessage it just scrolls right back up to the first picture. It won't let me scroll through at all without going back up. Is there something I can do to fix this?