What could be causing a swollen groin lymph node after being treated with antibiotics twice and tested for pid(result- negative)and they didn't work?

Swollen node: Is an immune response to an infection, can be swollen due to a cancer( most likely not your problem), you can always be referred to an infectious disease doc for an evaluation and work-up.

Different treatment: How didn't they work? If they're still enlarged but not sore, red, or hot, then it could just be residual scarring in the nodes. These can take months to heal. If the lymph nodes are still tender, red or hot, then the infection may not have been cured. You may need surgical drainage or different antibiotics. See your doctor for follow up to be sure what's going on.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells/digested food. Lymph nodes "taste" the lymph everywhere for bad stuff like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.