Everything is going to be ok...really.
The "normal" chemicals in your body are rebounding from the absence of narcotics. As this occurs, they sometimes over-shoot their normal levels and some anxiety occurs. If this persists for longer than what is comfortable or seems normal for you, you might consider some professional help. Anxiety is a very treatable condition.
Please go to NA meetings, get a sponsor and work the steps. If this is not enough, get a therapist as well. You can put as many resources to work on your addiction as seems necessary. Just don't give up and go back to using - add another resource instead.
You can do it - but you can't do it alone. Immerse yourself in recovery and surround yourself with recovering addicts. That is how your not going to pick up a needle again. Good luck.