I have this nice mare, she is about 5yrs but really she is only 3yrs she got started late in being broke.?

the thing is I have been riding her off and on for two years, all the sudden now she wants to run out from under me or she when we are riding on trail and anything touches her belly she jumps out of her skin. this very unsettling for me and her. I have tried lots of different things with her to get her over this. at first I thought it was just me. but I am not the only rider she does this with. I also have been riding 40 plus years and have rode lots of different breed of horse and have train lot different horse. but this one has got me stumped I am open to all help or ideas. this mare is great on the ground and I don't think that she is doing this out meaness I truly believe she is frighten of something. she also has no history of abuse of any kind.