Diagnostic mammogram and ultrasoundis s

I am 62 and went for my yearly mammogram. I've never had any issues before, but was called back this time for a diagnostic mammogram because there is a mass that lookslike a lymph node. The radiologist could not see it on the ultrasound but did see it on the mammogram. He said it was very small, 5mm. He thinks it is a lymph node and told me to come back in 6 months for another mammogram. It is in the inner periareolar of my right breast. I’m concerned because if it is a lymph node, shouldn’t it have shown up on previous mammograms? Is the best thing to wait six months? That seems like a long time if it is something serious.