Enlarged Lymph nodes in Groin and neck.

So I have enlarged lymph nodes in my neck and a cluster of enlarged ones in my left groin.  I got an ultrasound because of some groin pain and the doctor saw the cluster and said the biggest measured 4cm X 1.5cm and said it was benign.  First of all, can he tell if its benign with an ultrasound?  Second, I don't have any other symptoms but the ones in my neck and pretty big.  Also i have one the size of a peanut M&M in both armpits.  My doctor ordered a CBC blood test and it came back normal.  I've heard you can still have lymphoma with no symptoms and blood tests being normal.  I'm not overly worried but I'd like to hear if I should seek a second opinion?  Thanks a lot in advance.

Best wishes.