I doubt it's the lantus, unless you drank it :)
My advice: 1- check your sugar carefully- if you have an infection, your sugar may go higher; if not, you may go lower from the diarrhea
2- no milk for now
3- drink gatorade for hydration
4- eat yogurt (with probiotics) or kefir
5- I would contact dr if: sugar becomes too high to control (300s) or if diarrhea worsens to the point where you can't rehydrate (see # 3) i.e you get light-headed or change in mental status or vomiting
Good luck and remember- life always has periods of ebb and flow