[Acne] lump left underneath skin after nodule pimple?

I got a very deep pimple around my time of the month, and it was a nodule I believe, not a cyst. It never came to a head, just a red, hard, knot that eventually just started retreated back. Now 10 weeks later, it is flat from the surface with a dark spot. But if I put pressure down on it, there is definitely a bump there. Not a huge one, but still there. Like someone just stuck a little tiny pebble under my skin. It has me worried it won’t ever go away or it will re inflame?

Does anyone know what this is or had it happen before? Is it scar tissue? Still a pimple with pus inside? Anyway I can encourage it to either come to a head or dissolve? It seems so far from coming to the surface though. It has retreated to where it’s flat, so I doubt it will come to the surface again. But will it eventually just shrink completely??

Thanks!