People say having flu symptoms is an early sign of pregnancy?

2 days ago I had severe flu symptoms but they went after a few hours, I was however left with bad back pain which I was provided with 'Naproxen' as pain relief. I'm currently 4DPO.

1. Can flu symptoms really indicate pregnancy or is that an old wives tale?

2. Is Naproxen safe in pregnancy? I'm scared to take it in case I am and Google has conflicting info!

Thank you!