Why do I feel like crying, when I am alone, for no particular reason?
You are experiencing the emotion of pathos. It is the anxiety of Affinity. Affection lost, affection in doubt, affection denied.
So, when you are not keeping your mind distracted by the details of the day, it is coming back to a memory or a loss and dwelling on it.
You are the only person who knows what this loss is, but like so many things we don't want to face, and put away from our thoughts, they stay right there at the edge of consciousness, waiting to intrude.
This speaks to a loss that is too painful, or too scary, or has other dimensions beyond just the loss, perhaps betrayal, or even a more damaging transgression that makes a person who you need to love, unlovable.
If you can, let the tears flow. If the images and events that well up in your mind's eye are upsetting, be with them a bit. And, then let go. It takes time to mourn our losses. If the memories are too upsetting, so that you shut them down again immediately, then perhaps you might want to see a therapist who can help you open this can of worms.. so that you can move on with your life.