Baby Grunting

My lo is 4 month and has now started grunting all the time. Not just at night when I put her to bed. It's constant and can be at random times, after eating, changing, when she's tired, when she's playing etc. I honestly don't know what it can be, sometimes is can be really loud where she tenses and arches her back! I've tried burping as I thought it may be wind, changing, raising the crib, nothing helps. She has never been like this before and I don't know what to do. Please help.