Lipoma removed, learnt is was a lymph node. How common is it to have this?

Thank you for sharing your question and experience and I am sorry that this has caused you to be concerned.  There are numerous lymph nodes within the armpit, often sacrificed for diagnostic purposes in cases of breast cancer and other malignancies.  They can resemble the same tissue quality as a lipoma which in either case should have been sent to the pathologist for evaluation.  The pathology report should not only confirm the diagnosis but discuss any causes of concern in the specimen.  I would discuss your concerns with your surgeon who is in the best position to offer reassurance. 

I can understand your concern but let me try to put your mind at ease.  Lymph nodes are normal structures and are part of the normal lymph system that runs throughout your body.  Everyone has lymph nodes in the armpit as well as in the head and neck, trunk, legs, all over.  LNs can enlarge from a variety of non cancerous conditions such as bacterial and viral infections like cellulitis or an upper respiratory infection, inflammation as in an insect bite, and other common conditions.  That being said, talk to your doctor.  Ask "Are the lymph nodes cancerous" and have him/her show you the pathology report.  I am sure it is fine, but ask, ask ask.  Dont ask.  You wont know.  Good luck