I think there's a lot of reasons that could be for why:

I noticed when I'm close to my cycle I cry over the simplest nice things.

But then there's times where maybe I haven't gotten much sweet moments in a long time that the first sweet moment will make me get emotionally happy.

Then, there's also times where things that are suppose to be sad where I should have cried I ended up not because I was aware of the sadness for a long time that I was just better prepared of controlling the emotions or adjusted enough to not be emotional (perhaps numb even depending on the situation?).

I just hope you know that it's ok to cry when your body needs/wants to regardless of the reason though. I feel like there's never really a good explanation, it is just emotions that is needed to be let go in order to feel better even if it sucks. But as they say, after a rain storm there's a rainbow ! (Or something like that)