Perhaps wondering what's stressing you out is stressing you out. Talk to your doctor, and tell him/her that (s)he is the cause of your stress. Demand a refund.

Try getting rid of the stress?

no offense intended, but if you cannot think of a single thing that causes you stress then something is either very wrong or very right, probably the former. Just working causes stress, although not as noticeably as you think. When people think of stress they think of things that just frustrate the hell out of them, but in reality most stress is more subtle. For example, maybe you have a quota you have to meet everyday and work hard towards it? Maybe you're displeased with your life? Maybe you've been noticing how much weight you've put on, etcetera. All of these things are considered small and part of everyday life, and yet they can affect your behavior towards others. Try keeping track of how different factors affect your mood.

First of all, is you can't think of any couse for stress, you should discuss this with your doctor to find out why then he felt this way. Sometimes the source of stress is repressed or might be related to something that happened in the past, perhaps long ago. Things that were troubling or traumatic are offen repressed by the mind. This may or may not work, but sometimes you are able to think of repressed thoughts if you go to some quiet place where you can be alone, and you just shut out, little by little, everything that is in your mind. Allow your mind to go blank. First of all, you will have to turn off all those little voices in your head that are always putting thoughts into your head. Once you start concentrating, those little voices will be busy and your head will be buzzing with thoughts. Get past all of this interference until you are able to clear your mind, then just keep still, don't try to think of anything, keep your mind perfectly clear. This might take a long time, probably over 15 minutes, maybe even a half-hour. Just be patient, and try not to fall asleep. If your mind has something buried that is casuing your stress, this might allow it an opportunity to be discovered. We did this as an exercise in a class I took, and I was amazed how well it worked. If not, keep talking to your doctor about possible other causes.

First I would go to another internist. If he gave the same assessment, I might see a therapist. However, my money is on a different diagnosis.