loss of appetite and nausea

I'm a 63 year old male...usually excellent health...runner....last week i began having feelings of nausea along with loss os appette.  Had an upper GI which showed up nothing unusual except a great deal of acid reflux even though i've been on nexium for years. My GP told me to double up on the nexium for a couple of weeks....have done that but the symptoms are still there.  Guess i'm scared that i might have a serious cancer.  I have no other symptoms othere than loss of appetite and nausea....GP gave me something for nausea but it hasn't helped.....any thoughts? Thanks