I'd guess you're right about it growing heavy with eggs and then either releasing or reabsorbing them.
I'd take the fish back to the store you got it at. They need a reminder that they need to tell their customers when getting fish like this, or the arrowana, or the pacu, that the customer needs a gigantic tank. If they won't take it, see if any
LFS can take it (and then write the original store a letter regarding this issue). You might be losing money out of the deal, but as that fish grows, it will start eating the other fish in the tank, and then it will die as it outgrows the tank.
I wish stores would stop keeping fish like this in stock. It ticks me off.