Why is my 2015 Honda Civic moving side to side when I drive?

When I drive 50+, I feel my car shifting side to side just a little bit. Is it because the lug nuts are not tightly screwed on?

Which direction you're going? North, South, East or West? Keep going, there's a Honda dealer nearby would honor the factory warranty.

There's something wrong and it needs to be addressed right away. What has been done to the car since it left your dealership? How many mile you have driven the car before the problem start?

Unlikely, but easy for you to check. Grab wheel brace from the trunk and give each bolt a tweak.

Tire pressure?
Wheel alignment?

If the car is new it's unlikely to be worn suspension components or bad shocks.

Or just the way the car is? Hard to say without driving it. Get the dealer service manager to take it for drive (at highway speeds) and see what he thinks?

Simply sounds as though the steering responsiveness is much more sensitive than what you are use to. There's absolutely nothing wrong with your brand new Civic.

Loose lugnuts
new tires
bad alignment
loose parts

Front wheels might not be aligned or tires not balanced.

Brand new? Something you are not use to driving is a possibility. Wheels will be aligned, rims on tight. Is it perhaps "WINDY" where you are? That has a lot to do with it. See your nearest Honda dealership for them to inspect it for it is not safe.