Are swollen lymphnodes in abdomen and pelvis cause for concern if they remain swollen for weeks?

Yes: Yes if persistent. Especially if they are associated with symptoms such as fever, night sweat, weight loss, etc and no other causes to explain. In the younger age group, germ cell tumors can also be a possibility. Of course, they are many benign reasons. My point is that persistent swollen lymph node is a concern and should be evaluated by doctor.