Can hyper-inflated lungs be misdiagnosed, from you just taking a bigger breath than the last time you had an x-ray?

Hyperinflation is a specific finding and taking a bigger breath by itself would not make the diagnosis of hyperinflation. You should get a pulmonary function test to determine if you have obstruction. The most common causes of obstruction leading to hyperinflation are COPD and asthma. The difference between the 2 is the bronchodilator response. Good luck!