So I've never noticed this but my girlfriend did a couple of nights ago.

When I'm asleep I'll breath in deeply, hold it for a second or a couple of seconds and then as if I can't hold it anymore I'll let a small bit go and hold the rest. Thenths of a second later I'll let another small bit go, and I'll do this 2 to 3 times before completely letting all the air in my lungs go.

I don't always breath like this when sleeping, it just happens a couple times a night.

Any idea what's causing it? Or what I could do to stop it?

You should speak to your Dr. in regards to a sleep study. What you are describing is sleep apnea. Pauses in breathing while sleeping. It does have serious side effects and people do pass away in their sleep from this. It can be watched during the sleep study and if you are diagnosed with it. There is a mask that you can get that gives you the oxyegen that you require while sleeping callee a CPAP. Sooner the better for seeing your Dr.