Can you give a better idea of how hard your trainer was going? A lot of new guys overestimate the power their partner is putting into a shot. This is because new guys will walk into punches, or get caught while throwing. Both of these scenarios will make incoming punches more damaging. If your coach is rocking you with knees to the face as the introduction to your sparring career, though, I'm not sure what to think. I'm a big proponent of hard sparring (when balanced in a routine with slower-paced technical work), but that is excessive and pretty mean spirited if it was done in the context you've described.
Do you think you may have been tense and muscling out hard strikes without realizing it? With the warning on intensity and subsequent beating you got, in that order, my first thought was that you were just getting "settled down". If that's the case, just continue to focus on going as light as he said. This is probably the best solution to the problem either way. I find spazzes either tell you to pick it up, or just chill out (EVENTUALLY) once they pick up on the obvious difference in intensity if settling them down is not an option. Against your coach, it's probably not.
If the beating was totally unprovoked, I'd speak to the man about it and respectfully ask for a clarification of the sparring rules. Depending on his answer, you might want to reevaluate your stay at that gym. You have to train smart, try new things and learn as much as you can while minimizing the damage you soak up. The environment there sounds like a bad trade-off of risk and reward in your journey.