Breastfeeding and Possibly Pregnant?

My LO is 7 months old tomorrow and I’ve exclusively breastfed from the beginning. I’ve had periods since then, but never really kept track. Well, now I’m considered “9 days late” since my last one. I took a test yesterday evening and it was negative. I know FMU is what’s best, but I figured since I’m so “late” it’d be ok?

Well, now I’m just worried because I have heartburn, headaches, and nausea. Should I test again or wait it out? Anyone else have this experience?

Edit:

We are not TTC, but we also haven’t been using precautions very well.