I haven't heard of lymphoma triggering RA.  However, clinical studies have shown that people with RA who take TNF-alpha inhibitors (immune system inhibitors), such as Humira, Enbrel and Remicade have a slightly higher risk of developing one of several types of cancer.  Lymphoma is the cancer most often developed when taking these drugs.  Granted, the risk is still fairly small, but it is enough of a risk that all of these drugs carry this in their side effect information.  Other DMARDs like methotrexate also carry a higher risk of cancer.   One study recently linked methotrexate with melanoma.  However, I think most opinions I've read from physicians, researchers and even the FDA say that the benefit of these drugs outweigh the cancer and infection risk associated with them.