Hi YellowBroom.

I am not a doctor, but questions about lymph nodes appear regularly on this forum.

No, swollen lymph nodes are not always visible.  Just a couple of days ago I seemed to be fighting off a minor viral infection.  I could feel pains in my armpits and groin from some of my lymph nodes, but there was no visible swelling, and they were too deep to palpate.  In fact, I rarely get visibly swollen nodes, but sometimes I can feel that they are swollen to the touch.

The knot you feel is in the right position to be a lymph node. As you've only just noticed it, it may be fighting a minor infection you've picked up - so minor that you're not aware of any other symptoms.  I've had similar issues with a single node popping up. This is what they're supposed to do.

A single swollen node which is soft/rubbery to the touch, particularly in the absence of any other symptoms of lymphoma (such as drenching night sweats, itching, shortness of breath or unexpected weight loss) is unlikely to be anything too serious. 

The general advice is that one shouldn't worry about a swollen node until it's been up for more than 3 weeks. I should check it every week, and if it's still there after the third week, see your GP. Of course, if you should get any of the other symptoms I mentioned above then don't wait to see your GP.