Lymph node lump

Hi, I am 25 years old and recently discovered a lump in my lymph node.I went to my gp today and he has perscribed anti biotics and a follow up appointment in a weeks time.I know time will tell but until then I am worried sick, my family has a high rate of cancer, my grandma recently died of lung cancer,my uncle has bowel cancer, we lost my cousin to stomach cancer and my dad has melanoma, my grandad also has cancer in the groin. I have been unusually sick all year on and off with flu (blood tests later showed respetory virus), suspected asthma, colds,hives for no apparent reason that lasted 3 days.I am just wondering what are sure signs of cancer? The lump is the size of a 20c piece and is tender to touch.My gp checked other lymph nodes for lumps but there was none.I appreciate your advice.