It is likely that the way you are sitting combined with the duration of sitting, is putting nerves supplying the affected area under pressure, thus leading to your symptoms. This is why it does not happen when you are standing up or lying down. Measures such as getting up a little more frequently and walking around while you are at work might not be a bad idea. I would though also strongly suggest you see a Neurologist, who will be able to take a more detailed account of your symptoms and also perform a thorough examination.