Get a ruler and look for an accurate measurement. 1.5 cm is about equivalent to a small-ish peanut M&M, or about 3/4 of an inch.
Don't worry till they tell you to. I was told I had "lumps" and was sent for mammograms, repeat mammo's, ultrasounds, needle biopsies, and never had anything (the best they could come up with was a movable/changeable cyst). But I worried myself sick in the mean time, so take it from my experience...don't stress too much until they TELL you to.