Feeling out of breath while swimming and running?

I swim every week, and since about a year ago, i've been noticing difficulty holding my breath for a long period. When I stop, I need to take lots of deep breaths. Anyone any advice on this as I kinda need to hold my breath. When I have to even do it for a few seconds i start to struggle.

The thing with running, I run, paced, for a few minutes, then I get a majer stitch, but not only that, I get out of breath and start to feel dizzy and faint. Trouble is, when you run with school, pe, they don't want you stopping. I find that the only way I can get rid of the stitch is to grab hold of it, but sometimes thats not possible for me. Even when I run in a game, this still happens and I really need some advice. Also, can anyone tell me what causes dizziness and faintness on treadmills?

Any advice on these?

I'm 15, Female, about 5 ft 6 and about 95lbs.