What is the risk of lymphedema if only the sentinel node was removed during mastectomy?

Very low less : Less than 5 % . There will be an initial swelling of arm but subsides with elevation

With a...: mastectomy alone there will be a risk of lymphedema, with removal of a sentinel lymph node that risk goes up slightly (in my opinion probably negligible). So it's the mastectomy itself that increased your risk. Follow-up with your docs to detect it early. Remember post-op it will be normal to have some swelling. Hope that helped.