It sounds like your immunological system was already fighting something, which you succumbed to later. However, it also sounds like you have a long-standing problem which your immune system has been reacting to for a long time.
I always get a heavy white coating on my tongue, which is both lymph system and lung cleansing. I also have sensitive lymph nodes, so I have a similar problem, but not as bad as you.
I think the method that might help you the most are dry-skin brushing and detox baths. There is a detox bath forum in curezone.
The bath is described in the post, but not the dry-skin brushing. To do the brushing, you need a body brush with soft bristles, preferably natural rather than nylon or plastic, and it helps to get one with a long handle. Brush the entire skin surface of your body lightly, starting with the hands and feet and work inwards towards your navel. When you are doing the back of a joint, like behind the knee, brush in circles. Otherwise, brush in straight lines towards the center of your body. You`ll probably find that the areas with a lot of lymph nodes - behind the knees, in the armpits and under your ears - are itchy. Remember to brush lightly. This will move lymph towards the major lymph areas around your navel, where it has easier access to the kidneys. I believe there is a dry-skin brushing forum you can go to for more help.
Another thing that could help, long-term, are herbal detoxification of your lungs. My favorite is Mullein Lung Complex, but there are many on the market and any would be helpful. That helps me with post-nasal drip and coughing.
Finally, I think you should see an allergy specialist. People can develop allergies at any point in their lives, and they seem a lot like colds or even mild flu.
It is also possible that you are suffering from candida, which is a constant stress on the immune system. There`s a forum in curezone for that. You can get tested for that at the doctor`s or go ahead and work on it yourself.
You should consider doing liver flushes. There is a Liver Flush Support Forum if you want more information.