Confused Af 😖 Help Please

Haily • Moo 🐮
I had protected sex with my boyfriend, condom did not break we even check before & after, no holes no nothing! My period was 2 days late but I wasn't experiencing any early pregnancy signs
I took 3 pregnant tests after my missed period at different times in 2 week time frames, all negative!
But I now have back pain & my head feels really heavy during the day
What could be causing this?