Resin got hot – hardened quickly

Hello everyone; had a new experience maybe some of you have had before, but it was a first for me. I mixed the two exactly parts of epoxy resin (quart of each), then divided the goo between five cups. Then I started added mica powder as the color agent to three of them and acrylic paint to one and alcohol ink to the last. I needed this second pour to be more opaque than the first so I mixed in extra pigment. Once I mixed each color thoroughly, I let it sit about 15 minutes to thicken up a little so this final pour wouldn’t run as much. The material has a 50 pot time, so I thought I was fine. However, after 15 minutes, each cup was hot to the touch, nearly hardened and completely unusable. What did I do wrong? Help me to avoid this in the future. Thanks.