What is going on?

After I ovulated around sep 15/16 my back started hurting in thr middle lower region whether I was laying down or moving alot. I had all the symptoms of being pregnant except nausea. But got a bfn on the 28th and then the night of the 28th, what I believe was my period showed up. Normally when af shows up by day 2 it's heavy but today it hasnt been. Anyways my back is still hurting in the lower middle. Idk whats going on? Is it possbile a chemical pregnancy? Could I get a bfp in a couple days? Has this ever happened to anyone else before?