No. Puffers are egglayers. As such, they can NOT become "pregnant" because fertilization of the eggs occurs outside the fish's body.
If there were young puffers in the tank in the store, it's because they came in with the other puffers, just at different sizes. Little is known about the breeding habits of this species, and instances of captive breeding are exceedingly rare.
It's pretty safe to say that a male/female pair of puffers is not going to breed in a hobbyist's tank... But with only one puffer in the tank, I can say this with 100% certainty.
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