This is just good medical practice ... they are being vigilant and it's pretty much routine when "anything" is seen that requires additional views or testing. Be sure you do follow the recommendation of the 6 mo. recheck. Regards .....
If there was a lymph node prominent enough to be spotted (or what they thought was a lymph node), they probably want to keep an eye on it. Anytime anything on a mammo is anything other than COMPLETELY normal, they generally start being more aggressive with surveillance. My guess is, you'll go back in 6 months, it'll be gone, and your follow up will go back to normal!