Sometimes Have Double Eyelid?

It is very normal for your creases to vary from day to day. It is a feature that makes you unique. Now if you don't like it , it is possible to surgically create a dominant crease and this is referred to the double eyelid surgery. This should give you a single crease that will be present always. Good luck

It's part of what makes you an individual.  It's not uncommon for Asian to have variable eyelid folds (supratarsal creases).  If it bothers you, surgery is an option to create more permanent creases.  

It's a spectrum- some people have the crease, some people don't and some such as yourself have partial and intermittent. This is because of the difference in the anatomy- the skin attachment of the mechanism that open your eyes (elevator mechanism) if present will give you a crease and if there's only partial attachment you get a crease that sometimes show up especially if you're tired or have not slept well. The difference between both eyelids are also very common and you can have the surgery to address it if this is an issue for you.
Wish you the best in your journey.
Dr Leo Kim, Specialist Plastic Surgeon in Sydney