I agree with Dedicated. It's likely a combination of stimulant intolerance and doing too much, too early with no food. Try to have something light before you take the HRX; yogurt, a shake, whatever's easiest. Also, did you start with just one pill, to check your tolerance? I've seen people buy their first bottle, jump right to two pills twice a day, and then ask why they don't feel so good. Especially with fat burners, the max dose isn't always the best bet.
To the original question, I don't exactly feel more energy with HRX, but that's okay because I'm not supposed to. It's a fat burner, and I know that when I use it, it does makes my fat loss program more effective, most especially in killing my appetite. Most clients say the same thing. Appetite - gone.
If I want or need an energy boost, I go for Spike or a Spike Shooter.