Can straining on the toilet cause pelvic lymph nodes to swell?

Lymph nodes become larger when they are activated to filter serum of debris. This is most often seen during immune activation and illness. I can find no plausible reason straining at the toilet would trigger lymph node swelling at any location.

Straining to pass stool would not cause enlargement of lymph nodes. For constipation - You may take a fiber laxative like Metamucil and drink enough water daily so that your urine is colorless. For the long term you may try to increase your intake of fiber foods. For info: http://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/high-fiber-foods/art-20050948