What type of sensations might you experience?
You might feel a sense of being at one with the universe, which in a way, is the opposite of concentrating on your body as you will feel as though your consciousness does not reside in a physical form.
You might feel lightheaded, or experience a tingling sensation. Some people report that they have involuntary movements such as a hand rising, while others say that their body twitches.
Basically, you might have a wide variety of physical sensations, or none at all. Either way, the very act of relaxing will be beneficial.
Why do people have sensations?
The jury is out regarding why some people experience sensations while they meditate. Some teachers of meditation explain the phenomena by saying that they are caused by the release of stress as part of a healing process that takes place. Others say that sensations occur simply because the brain is relaxed. What we do know, is that people tend to become more aware of physical sensations while they meditate. Instead of focusing on the outside world as they normally do, they pay attention to their inner-world, including their body. If, as you meditate, you concentrate on the process of breathing, or on your heart area, or perhaps chakra points, you will pull your attention in the direction of how your body feels. You might experience an awareness of sensations that regularly occur, but which you do not usually notice when not meditating.
What should you do when you have sensations?
If you experience sensations, you might like to imagine that you are breathing into the areas of your body in question and make doing so part of your meditation practice. Alternatively, you can just allow them to happen without paying them any more or less attention.
It can be normal to discover that you become aware of your body and of specific sensations while you meditate. Instead of letting them worry you, you can make them part of your practice purposefully, or allow them to simply happen.