Hi Serene,
Don't worry too much about it. Probably trying to suit into new environment.
It's good to give your hamster a healthy mix, variety of food. Hamster food / mix are great, but you also can prepare your own mix.
Since you noticed that the pet shop gave oats as their (only) meal, your hamster might be having trouble adjusting.
It's usually advisable to mix (introduce) new food gradually. So, if you want, you can get some oats to see if there's any improvement in it's eating habits.
Give your hamster some time, I'm very certain it'll "recover" on it's own, since you've provided both food and drinks at it's disposal

About hamsters (and gerbils) and their teeth and nails:

Something that I used to do, is provide some "driftwood".
The kind which you put in an aquarium - either mango or jati, the two most common driftwood that you can find.

They love to sleep on it, they love to burrow under it, scratch it, bite it, jump from it, crawl... very subjective...
Basically, it's a wonderful play thing.
Not to mention the aesthetics if you found a darn good looking driftwood!

The only drawback is the cage itself.
Driftwoods would be more suitable if you're keeping your pets in a tank.
Smaller driftwoods may fit smaller cages, but will provide extra height for your hamsters to be more adventurous, and it might be dull looking...

Anyway, back to topic, driftwoods are good alternative means for teeth and nail (grooming).
They absolutely love it. Give it a try!