<human>: Could you distill the essence of the text into 1-3 sentences?

She and I are kind of similar in this way, but some days she is doing great and having a great day, and then, out of nowhere, she will get semi-depressed. She doesn't want to do anything, she doesn't want to get out of bed, or go anywhere. 

I ask her if anything is wrong or if she is upset, and she says no. 
I believe her. 

Like today, I wanted to go do some stuff about town, and she just didn't want to do anything, or go. Which, I have moments like this too, so I understand. But I also want to make sure I treat the situation in the best way possible, and make sure that I'm doing things I want to do, but also make her feel involved when she is having her "down day." 

So:  how do I treat her when she is having these down days, and is it wrong to try to cheer her up? If it's okay to try to cheer her up, how should I do so? If not, what do I do otherwise?
<bot>: Girlfriend gets depressed at random moments occasionally; is it wrong to try to cheer her up? Or should I do something else, and give her alone time?