Hi Sarah
This dramatic drop in your bg levels may just be a 'one off' and might not happen again. However, so that you can find out if your bg levels start to change direction, set an alarm to go off on your mobile phone about 2am and then get up and test and if necessary, eat some carb (about 10g biscuit) and then get up again about 5am and then test again. If the bg level drop starts to have a pattern, then you will need to reduce your Lantus down a bit.
I had to reduce my Lantus dose from 16u down to 12u over a period of 2 years and then after that, I then had to increase it up to 19u.