Help me

Hey girls,

I had my last period on 30 of July and it finished on 3rd of August.

I had sex on 11 August but he didn’t came inside me, my period supposed to be on 23 of August but I didn’t get it yet, I’m 8 days late and I’m having backache.

I did 2 pregnancy tests, one of them was on 25 of August and the other one was on 28 of August. Both were negative.

What do you think is going on? Am I pregnant or what’s that? Also I’m stressed 24/7 I don’t wanna be pregnant !