Hi Andrew! What was Sunny's BG when you shot? Sometimes a little can leak out, or there could have been a little insulin on the outside of the barrel from when you adjusted the dose. Insulin has a very strong smell and a little goes a long way.
Having a cat go that low so soon after starting Lantus is unusual. The shed usually take 5 to 7 days to build up and until that time you don't see the full effect of the dose. I agree w/ the others that 1.5 u would have been safer, but since you shot a higher dose be prepared to keep testing and
be ready to intervene w/ higher carb food or syrup/honey. Ice cream works too (and you can give yourself a treat at the same time

), anything w/ lots of readily absorbed sugar. This could be another very active cycle or Sunny's BG could bounce from getting into number his body isn't used to. The liver can "panic" and release glucagon and counter regulatory hormones to counteract what is perceived as a hypo. The body is used to nigh BGs and sees normal numbers as dangerous.
FD is a marathon, it took a while for it to develop and it takes consistency and patience to reverse those effects. It is far safer to start at a dose that might be too low and work up to a good dose, than to start too high and risk hypoglycemia. Hypos can come on suddenly and are much more dangerous to a cat than a relatively short time in higher numbers.
You are also now in a situation of shooting 4 hours off from you preferred shot time. As Wendy mentioned, Lantus likes consistency, in
timing of dose as well as the amount. You can readjust your shot time by 15 minutes a cycle or 30 minutes a day. However, if he goes quite low again and you decide you need to skip the shot, you can reset your shot time to when you want it to be. Usually when we stall for a low preshot number, after 2 hours we just skip the shot. This will drain the excess in the shed and allow you to get back on schedule, otherwise it takes too long to get back to the preferred shot time.