Severe stitch while sleeping

I think this is really strange & I don't think anyone will think they are linked, but pregnancy & stitch. During my pregnancy with my 2yo I got the first one just before I found out I was pg while I was sitting watching TV. I had about 3 or 4 more during the pregnancy & one whan DD was about 8 weeks old. I am 15 weeks with this one & have had 2 already. They always start during the night. The only one that didn't start while I was sleeping was the one after I had my 2nd DD. They hit me so hard I can't move & can hardly breath. It is so panful all I can do is lay on my right side trying to breath through the pain & tears til it goes away. It usualy lasts about 15 minutes to 1/2 an hour.
Dos any one have any ideas why this could be happening? I've tried looking it up on the net, but everything said about getting a stitch has to do with exersice. I'm not exercising when it happens though!
Any ideas at all?