What may be happening here is that the Lantus depot takes a few cycles to fill up; when you were shooting only once per day it really didn't get a chance, but you just did three cycles in a row of 0.5U, so now the depot is filled and you're able to see what this dose can do!
100 and 120 are quite safe numbers, by the way, and experienced lantus users will shoot such numbers (and lower) all the time. We just don't recommend starting off shooting that low, it takes a little while to learn your kitty's patterns.