"started by chanting for the earth's core to bring me to her/him by astral projection and started to feel myself being pulled downward towards the core"
I think it's quite obvious. You called to something very vaguely that you know nothing about, and gave them permission to drag your consciousness (and part of your energy/spirit) to them.
Not smart. I have no idea what or who they were. Since it was down, it could be a number of beings. It could be figurative, as in the lower worlds or infernal, or spirits of whatever our core is made of. Science can only guess and doesn't know for sure what is down there. Recently they found rocks that were made from water. It's scientifically mind boggling but is making some believe that there is a single very huge mass of water that is under our earths crust. They just figured this out in July of this year! Water, as we all know, is life sustaining and the oceans are yet to be entirely explored. So there is still a lot of mystery to our planet and who knows what more we may learn in the coming years.
Please refrain from asking vague and unknown beings to do things for you. By appealing to the earth's core as an individual you are asking for something with intelligence. But you gave no name, no description, and I'm sure you didn't put any thought into what the earth's core could be associated with. So it was an open ended request that was about as safe as hitchhiking and letting a random stranger drive away with you.
I'll look at just one fine example. Good old Greeks. The deepest depths of the earth was Tartarus, a prison for dangerous beings and the worst of the worst. It's said that the distance from the heavens to earth was the same distance (down) as Hades to Tartarus. The distance of the earth's surface to its exosphere is in accord with the distance from its surface to its core. It isn't exact, but if Tartarus is the core, the "heavens" are located in the middle of our exosphere. Coincidence? Perhaps. But it's something to consider when asking for parts of our world to do things for us. What do they get out of it? Did you offer something for the service up front? I can guess why beings of Tartarus would be more than willing to oblige without a price already laid out.