Liver Mets with undetermined primary site

Hello everyone, new here.  I have mets to my liver, however the primary site hasn't been found yet.  Suspect is colon, however nothing has shown on the pet scans to confirm that.  I had prostate cancer 13 years ago and had lung cancer almost 2 years ago.  I had radiation only for a small single nodule in my right lung.  Pet scan at that time was also clean.  After the radiation treatment I had ct scans with contrast every two months until last Sept.  All clear.  So they decided to scan every 4 months.  Last ct scan Feb 2019 showed multiple liver metastasis prompting another pet scan and biopsy.  Biopsy results showed metastasis, but not from the lung.  Hence the possible colon as primary.  I have had a colonoscopy done in 2007, 2010 and 2015 all clear.

So here is my questions.  How is it possible to have mets to the liver in just 4 months.  And the size of the tumors are too large (4.3 cm) and too many for either surgery or radiation.  So I am left with chemo only.   I will still have another colonoscopy and endoscope to try and find the primary site.  But how is this even possible?  I didn't think colon cancer was this aggressive.

If this is posted in the wrong section, please forgive me.  As of now, all I know is that I have secondary cancer in my liver, so this is where I started.