What Does Slightly Enlarged Lymph Node In Left Side Of Neck Indicate?

My name is Thomas. I am a male, almost 20, and I cant help shake this fear that I may have a lymphoma. There is a painless, moveable, slightly enlarged (pea sized) lymph node on the upper cervical region on the left side of my neck. It came up somewhat recently a little more than a week ago. I have had friends my age tell me they have had swollen lymph nodes like this that went away after a few weeks. But I do not feel sick. I sort of feel like there is a tightness in my chest sometimes, but my spleen appears to be normal in size because it makes a hollow sound when I hit the lower left side of my rib cage. I cannot tell if i feel somewhat weird genuinely or if its just paranoia. I don t feel that tired, I mean I still go to the gym and run and I do not have a Pel-Ebstein like fever. Although I eat healthy (lost 80 pounds) and do a lot of cardiovascular exercise, I have to admit that I have smoked cigarettes from age 16 to yesterday (I ve decided to quit in light of this situation) and I have done drugs and drank alcohol. I eat a lot of chunk light tuna, around 6-10 cans a week. I have learned that mercury poisoning can lead to swollen lymph nodes.My doctor says it is nothing to worry about because it is small and moveable and that if it gets bigger than 1 cm for over a month I should come back to get tests done. I DONT WANT TO WAIT. Lastly, I got shoulder surgery August 29 2010 and had blood tests done two weeks prior. If I had any cancerous tissue would that have come up on those blood tests?Thank you for all your help.