The last few times I've done my cardio, after about 20 minutes, I get this awful pain in my sides. A couple of times I have had to stop what I was doing and switch to just doing weights because it hurt so bad to do anything else. What is this and how can I prevent it? It stinks to have to cut workout times short.
It could be lots of things. If it's mostly on the right side, it could be the typical "stitch" pain that runners get when they're winded. If that's all it is, there's a simple fix: taking nice deep breaths from your diaphragm, the kind where you see your belly move in and out as you breathe. If that doesn't help, then maybe you ought to see a doctor to rule out anything serious. If it's mostly on the left side, you should definitely see a doctor to rule out heart problems.
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(I'm not a doctor, and this doesn't constitute medical advice.)
I used to get those a lot (when I used to not be able to breathe through my nose, not sure if that will make a difference). I found instead of trying such intense cardio at first, I started a lot more gradually. I can do things now just breathing heavy that would have had my side hurting a couple months ago.
It's likely that you're working too intensely. It's not known for sure what exactly causes those symptoms, so I have no definitive suggestion with respect to prevention. However, if you work less intensely you'll be able to go longer before that pain occurs. It usually goes away as fitness improves, as it did with the poster above. Of course, for a proper assessment and diagnosis it would be wise to check with your MD. Good luck sorting it out.
Apparently you shoudl breathe in through your nose and out through your mouth when you get a stitch. Doesn't work for me! When I was running the other day, I tried pressing the painful area and that seemed to help!
Also, how close to exercising are you eating? I had a snack before I went out and got a stitch, so perhaps you could experiment with that too.
I've tried breathing through my nose and out through my mouth and it didn't work at all.
It probably won't, if your breathing is still shallow. The important thing is to breathe deeply, so that you see your stomach (not your ribs) push out with the inhale, and in with the exhale.
Pain in the sides?
When I was running the other day, I tried pressing the painful area and that seemed to help! 
