More information on the type of bike you have and maybe a video of the grinding would aid in diagnosis.

As Argenti says - the easiest thing to do is to take the bike back to the shop if that's possible.

I'll take a shotgun approach.  
I'm going to assume you had a bike delivered to you in a box and did the final assembly.

If you have a front and / or rear derailleur:  
Things to check for intermittent grinding while pedaling:  
- front derailleur rubbing on the chain EDIT: no front derailleur on your bike
- rear derailleur not adjusted correctly EDIT: This is most likely issue  

EDIT: If you can get the rear wheel of the bicycle off the ground, pedal with your hand and listen for where the sound is coming from. My guess is that it's coming from the rear derailleur.

If you have a bike with a chain guard the chain might be intermittently rubbing on the chain guard. EDIT: You have a ring type chain guard that usually does not rub unless it's badly bent.

If you were missing bearings in your bottom bracket you would be getting constant grinding while pedaling and the bearings would be larger than 2mm. That size sounds more like a headset bearing.


The bearings are probably not related to the intermittent grinding/friction issue. 

If the bearings do belong somewhere on your bike you need to figure it out before serious damage occurs. I've seen some strange things show up in a box along with a bike and it is possible that the bearings don't belong to your bike. 

There are four places bearings should be on a bicycle:  
- Front hub
- Rear hub
- Bottom bracket - crank EDIT: looks like a sealed BB so it should be fine
- Headset - where the handlebars and fork come together  
See the [Terminology index][1]

On a new bike bearings are usually adjusted too tight. It's best to pull the wheels and check hub bearings, they should spin smoothly with no play. Slip the chain off the crank - it should spin smoothly with no play. Pick up the front of the bike and check that the handlebars move smoothly with no play.

That's all I can think of but hopefully this will get you pointed in the right direction.  
More information or a more specific question will enable a more specific answer.


  [1]: https://bicycles.stackexchange.com/questions/244/terminology-index-a-list-of-bike-part-names-and-cycling-concepts