Hi Danika,
I’m so sorry this happened to you. How frustrating!
When resin and hardener are mixed together, a chemical reaction occurs that produces heat. This heat is what allows the mixture to cure solid. For some reason, your resin mixture heated up too quickly. In your case, one of two things happened:
1. You mixed more than the recommended mixing amount. I have not used ecopoxy and don’t know what they recommend for their minimum and maximum mixing amounts. I would certainly check in with them to see what they suggest.
2. Something else, besides the hardener, sped up the reaction. This could come from the colors you added or even a reaction with the cup itself. (Certain plastics can do that.) If you got #1 right, then this is the likely explanation.
Let us know what you try next time!