I've been getting similair to you from about 20+ weeks.
For me it's just everything getting squashed upwards that's doing it, especially if I sit reclining or lie down in bed not long after I have eaten.
My midwife said it's nothing to worry about just not much room left.
I also get a racy heart at the same time.
Try sitting up or lying with a couple pillows this helps me alot when it happens and will usually get better in about a half hour.