The symptoms you describe are consistent with recurrent and repeated colds, but it is good that you have been to your doctor to eliminate a more serious problem.
Usually, such problems arise because your immune system has been under pressure from a variety of sources, including stress, worry or simply doing too much at work or at home.
The recommendation of vitamin C by your doctor is a good one and, you should also consider using Echinacea (if you are not already doing so) as this herb works by building up the immune system.
In the longer term, it will be good to identify what might be causing your immune system to work less well. Apart from the external factors mentioned above, you should also have a look to see if you can improve your diet. For more information, read the immune system pages on our website.