Weight loss can be caused by LOTS of things, not just stress and not just
lymphoma.  I wouldn't assume anything at all.  It sounds like your little
one needs to see a vet.
 
Is there any diarrhea?  Loss of appetite?  Our Pertwee lost a lot of
weight due to heliobacter (also had diarrhea) and then lost weight again
with no other symptoms.  It turned out to be the first symptom for
adrenal disease.  Several years later he lost weight again without
diarrhea.  It turned out to be chronic pancreatitis.  All were treatable,
but all were treated differently.  You will need your vet to figure this
one out.
 
All the best,
Caity and the amazing eight
[Posted in FML issue 4367]