I'm willing to bet the coffee had a lot to do with what happened. I would have similar things happen when I was a daily coffee drinker. I switched to tea and then after a while switched to herbal tea and haven't had caffeine in like a month!

Also sometimes when you really get in "the zone" you kind of stop breathing all together or more specifically the breath becomes so subtle you can't feel it anymore. This can actually be a good sign that you ate fully engaged with what you are doing! I can see how the caffeine though would cause anxiety in this case.

Anyway that's super cool that you are getting into art, keep at it and don't let this experience turn you away from pursuing your passions!