景気 is not just a condition but a (good) economic condition/climate. にわか is "temporary". にわか景気 refers to a sudden but temporary boom in business/economy, such as the Gold Rush.
私のにわか景気 sounds funny to me, and it's intentionally so. It's for showing how the wordphrase "my boom" sounds weird to the Western ears. In other words, 私のにわか景気 should not make sense.