You always want to push yourself, each time a little more, but you should not go so hard that you're excessively tired. I know sometimes I've worked out a particular area to hard, or tried to hard after a period of inactivity, and strained muscles. The key is to start slow, and gradually work yourself up.
Yeah, I hadn't worked out in the last two days, and I also woke up earlier then I wanted to today. That was probably it.
Potato.