Hi Emem,
If you are suffering from depression, you can be sad for no reason. It just happens. I would recommend that you talk to your doctor about this, as you could be suffering from depression at this time.
Often people just get the blues and it passes, but if you are sad most of the time, you could have depression.
Talk to your doctor or see a counselor and see what is going on.
Best wishes to you, and welcome to the forum.
Hugs, Karen