Like what do they mean by bad days. do they get weak and stuff.

the longer you have it you start to get problems with circulation and suck, possibly they ate too much sugar and spiked they're blood sugar or went to low.. etc. when your blood sugar is not stable around 4-5 it effects your energy levels.

depending on how well they control the glucose levels, they can feel bad, sick woozy, etc if they go low or hypo and have to do the sugar eating thing without proteins to balance it.

They can skip their insulin so they can lose some weight and go high then they feel like cra*!

Either way they don't feel good at all.

Diabetes is very hard to control. It takes a lot of diligence. It is even harder for type 1's. Everything you eat and drink or don't eat or drink affects your blood sugar. In type 1's their body no longer makes insulin, so they must be diligent about counting their carbs at every meal, checking their blood sugars and then giving themselves the right amount of insulin. And even with all that work, you are never guaranteed to have it just right, so sometimes your sugar can go to high or too low. It is a delicate balance and if you go out of your target range too much in either direction, you will feel sick, tired, grumpy, etc. So sometimes it is just a "bad day" Not to mention it is a lot of hard work everyday to be diabetic and you get no break from it, so it could be that they are just having a bad day emotionally and that too will affect your blood sugar. I hope that helps you!