Hi, I'm not sure if I'm just psyching myself out or pregnant, but I am freaking myself out a bit.

I have 2 kids already, age 4 and 1. DH and I aren't trying to get pregnant, but we aren't preventing it either. I don't track my period so well so I don't even know if my period is late yet. But DH and I had unprotected sex on Feb 25th, and at the time, I thought "Hmmm, I think it's been about 2 weeks since my period started, so something could happen here". So of course since then I've been on high alert and wondering about every "symptom" I have.

For a while, I thought my boobs were a little sore, but they aren't now. But that also could have been all in my head.
My lower back has been very sore since yesterday.
A few days ago, my hips were achy.
When I first wake up, I can feel how empty my stomach feels and feel like I should eat, but I don't necessarily feel hungry. Normally I don't eat breakfast until I have been up for a while.

Two days ago (the 14th), I couldn't stand it anymore so I took a pregnancy test, and it was negative. I figured that since it had been about 18 days since we had sex, it would have been positive by then if I was in fact pregnant. But it was negative. I did it in the middle of the day, not in the morning with concentrated pee. I realized that I was relieved to see that it was negative.

As of tomorrow, it will have been 3 weeks since we had unprotected sex, so if I was correct in thinking that my period was about 2 weeks before we had sex, my period would be about a week late. But if my estimate of my last period was way off, like it had only been 8-10 days since my period, I think my period would still be a tiny bit late. Or maybe I will get it today.

So is it possible that I took the test too early on the 14th? Like if we had sex on the 25th but didn't conceive until a few days later?

Thanks in advance for any suggestions/ideas. I know I should just wait and take another test but I just can't concentrate on anything else right now!