Hello.
First of all, the panic attack may not have been associated with the smoking. But when it does happen, it is normal to believe it could be due to the Marijuana. And if the panic attack was actually a panic attack as per the criteria, it can occur any time.
Marijuana (Cannabis) is not known to produce physical withdrawal symptoms and there is no physical dependence. But there can be a psychological dependence. And the anxiety can be explained. However, the anxiety cannot be an isolated occurrence without accompanying thoughts. You seem to have developed an unhealthy anxiety about your health. This is evident from the fact that you are looking to find out what can be wrong with a hint of obsession.
You may need to take efforts to reduce symptoms of your anxiety. You can take anxiolytic medication with your doctor's prescription. Often that is enough to tide over the withdrawal phase. Counseling is also known to help during the few weeks after cessation of Cannabis use. It is also important to "feel OK" that you have the anxiety. Do not let the secondary anxiety (anxiety about having anxiety) affect your ability to focus on reducing the primary anxiety.
Hope this helps. Take care.
Abhijeet Deshmukh, MD
(This post is for the purpose of providing medical information and is not a substitute for a face-to-face consultation with a doctor. This post is not intended to give or rule out a diagnosis, create a doctor-patient relationship or replace an existing one. I am not able to diagnose medical conditions online. Please consult with your doctor or a qualified healthcare provider for diagnosis and treatment options)