Dad goes for results of 3rd scan tomorrow. Really Nervous!

Hi Everyone, My dad has stage 4 CC with multiple lessions on his liver.  Tommorrow we go to get the results of his scan. The last 2 times they did a scan, it showed that the lessions are shrinking some from 3cm to 2 cm and his CEA went from 360 to 17 in July.  For some reason I am really on edge this time.  Not sure if its because they have not tested his CEA in a while so I don't have that to tell me that the chemo is still working.  I just hope they are going to give us some good news tomorrow.  I can;t sleep and I feel really nervous.  My dad still looks really good and works everyday.  The nurses always comment that he does not looks like he is fighting stage 4 cc.  Also on this scan it showed he had a clot on his lung and that scared not only me but my dad.  Is it common that Chemo will stop working after a while.  If that is the case do they try another combination?  Thanks for all your support.  Christine