Relax, no big deal, I often see airliners zipping by in the opposite direction when I fly. There are highways in the sky just like the ones on the surface. If you stare out the window long enough you’re bound to spot a plane going the other way.
Thing is, I am the least white-knuckled passenger I know. And I am not freaked out or anything. But my heart did jump and thumb a beat when I saw it.