I am only recently diagnosed as diabetic but I am a long time low carber. It is not uncommon for people to start off well, and then what is called 'carb creep' begins, slightly larger portions, eating things which look similar to the low carb ones but not when you read the side of the box. I have even had a product which was in the same design of packaging but with a carb count 2.5 times higher, entirely unannounced.
If it is not carb creep then perhaps changing the type of exercise you do, swapping to something different can help both with the enthusiasm and duration of sessions.
One thing low carbers tend to do is drink more, as the way of eating is diuretic. Are you drinking more milk? I have always been careful to drink a set number of cups of water each day.