I know it's easy for me to say but don't worry, cysts are very, very common and more common that you think because most of them don't show on mammograms, only in ultrasound scans.
I have had cysts and they are nothing to worry about but they can sometimes skew the mammogram reading hence the inconclusive report. The clinic I go to gives you ultrasound straight after a mammogram which is more accurate and shows up much more and immediately confirms any uncertainties. I have a mammogram every other year and ultrasound every year.
The specialist told me that in his entire career, he has never seen a cyst become sinister and if it did happen, then it was not a cyst in the first place. Also, cysts have nothing to do with taking HRT and apparently they come and go all the time or sometimes you can have them for years.
I hope they give an ultrasound and not another mammogram but good luck and please let us know how you get on.