Whenever I sit for a while, with nothing to do, a weight starts to drop on me and I feel like I could cry.

The only thing that keeps me from being sad is being up and doing stuff. So, I run back and forth through the house. I talk to myself loudly and try to distract myself in whatever way possible. But soon my legs start hurting or I get out of breath or I feel I need to lay down and I start feeling sad. Or I'll do it so much I get tension and then BAM! I drop just like that.
Just yesterday I was up. Laughing. Happy. Running around. Talking to myself. Yelling out loud. Having fun. Then suddenly I got really stressed. I started screaming and then I went into a full blown breakdown.
I need other ways to cope with this. And keep myself from going to the bottom.
Any ideas?