The "falling to her side"--does it look like she's rolling around like cats do to scratch their backs? Or does she look like she absolutely can't stand up and is falling because of that? Does she look like she has any control over when she "falls"? Is she eating and using the litterbox well?
Unless she really seems to have lost control of her body, I'm suspecting she's in heat. Cats in heat will yowl and cry and roll around flirtatiously. It does look very strange to people who have never seen it before. I would recommend getting her in to be spayed as soon as possible, or there will be a lot more cats for you to take care of ($$$$!) in a couple of months. You should be able to find a low-cost place to do it if finances are a problem.
If she's not eating, or is having any litterbox issues, or is falling with no control over her body, she needs to see a vet immediately. If you can't afford that, surrender her to a shelter so they can see if anything can be done, or put her to sleep so she won't suffer.