We report measurements of the in-plane magnetic penetration depth Deltalambda(T) in single crystals of ErNi2B2C down to ˜0.1K using a tunnel-diode based, self-inductive technique at 21MHz . We observe four features: (1) a slight dip in Deltalambda(T) at the Néel temperature TN=6.0K , (2) a peak at TWFM=2.3K , where a weak ferromagnetic component sets in, (3) another maximum at 0.45K , and (4) a final broad drop down to 0.1K . Converting to superfluid density rhos , we see that the antiferromagnetic order at 6K only slightly depresses superconductivity. We seek to explain some of the above features in the context of antiferromagnetic superconductors, where competition between the antiferromagnetic molecular field and spin fluctuation scattering determines increased or decreased pair breaking. Superfluid density data show only a slight decrease in pair density in the vicinity of the 2.3K feature, thus supporting other evidence against a pure ferromagnetic state in this temperature range.