Hello, I'd like to help you with your question.
Health Anxiety (what used to be called Hypochondria) is when you worry excessively about your health. As you mentioned, you are aware that health anxiety is a problem for you. Being concerned about smoking 5 cigarettes then would make sense, given your symptoms and history with health anxiety.
Most people are concerned for their health at times. People worry that a certain symptom may mean something more serious. But for people without health anxiety, they focus on the concern and react in certain ways- they may see their doctor to rule out any problems or they might instead put the concern in the back of their mind. If other health symptoms show up, they may see their doctor. Others ignore symptoms all together. But when you suffer with health anxiety, ignoring your health becomes impossible.
It's important here to put your concerns in perspective. You mentioned that you take very good care of yourself. That is a huge plus. So overall, your health is probably good if not excellent. Smoking a few cigarettes and any possible harm they could do balanced against the things you do to take care of yourself makes it highly doubtful that a few cigarettes could have hardly any effect. And that effect would dissipate quickly. There is simply no evidence that a few cigarettes impacts a person's health that drastically. Most people in their lifetimes smoke at least that much, most likely as curious teens. If that were an issue, we would know about it through the medical profession. Even a lifetime of heavy smoking, while it has effects, may not shorten a person's lifespan by that much. Looking at the facts can help you change how you see your actions.
Health anxiety can develop from different causes. You may have been traumatized in the past through a difficult childhood or a traumatic event. Or you have been going through a lot of stress recently or the loss of people close to you. You may also have a tendency towards worrying in general and you have focused on your health instead of something else.
Finding the cause can help you understand why you feel as you do, but resolving it is the focus. There are many ways to help yourself overcome health anxiety. The best way is therapy, specifically CBT (Cognative Behavioral therapy) because this type of therapy challenges your thoughts and helps you change them. Medication can also help, but what you may find is that it reduces your anxiety for a while but once you stop taking it, your anxiety comes back. So if you feel you cannot function right now, medication can help you until you can work on this through therapy. Otherwise, therapy is the best treatment.
You can also work on health anxiety through self help. It takes time, so be patient with yourself. You have to undo months or maybe years of thinking this way, but with time and practice, you can overcome your anxiety. Here are some resources to help you:
It's Not All in Your Head: How Worrying about Your Health Could Be Making You Sick--and What You Can Do about by Gordon J. G. Asmundson PhD and ***** ***** Phd
The Anxiety and Phobia Workbook by Edmund J. Bourne
Overcoming Health Anxiety: Letting Go of Your Fear of Illness by Katherine Owens PhD and Martin Antony PhD
Also, consider joining on line forums for people with anxiety or health anxiety. While you want to be careful to join ones that work towards feeling better and don't just talk about their fears, seeing others with the same anxieties and how they handle them can make you feel less alone and give you ideas about how to approach what your own fears.
Most of all, realize that you have good insight into your anxiety which is a sign that you can get better. Having insight and the understanding that this is an emotional issue to work on and overcome is healthy. And reaching out for answers means you have the motivation to get better which is the most important factor in recovery.
I hope this has helped you,