It was something perfectly ordinary: Lindsay Gottlieb, who coaches the University of California at Berkeley’s women’s basketball team, was about to fly with her 1-year-old son. But her family then had what Gottlieb called an “uncomfortable and hurtful” interaction with a Southwest Airlines employee at a Denver International Airport ticket counter: On Monday, she said, the employee asked Gottlieb to “prove” she was the mother of her biracial son, even after seeing the toddler’s passport. The employee then asked for her son’s birth certificate, citing “federal law” — and proceeded to ask the dismayed mother if she could prove she was the mother with a Facebook post, Gottlieb wrote in a tweet. “We had a passport that verified our son’s age and identity, and both parents were present,” Gottlieb said in a statement to The Post on Tuesday.