Sounds like your first day was...interesting. Like AB, I'm slightly concerned that you might've overshot the weights a bit. Like you said, conditioning might occur pretty quickly, and the perceived difficulty could've also been partly due to the week off.
Regarding warm-ups, their function, in addition to weight acclimation, is just what their name implies. Right now, you're essentially asking your body to go from a state of rest to a near-maximum effort.
Also, if your energy is good so far, I guess this might not be an issue, but you might benefit from more carbs before your workout.