I have a sliding hiatal hernia too, as well as Barrett's and GERD, and I get that massive pressure at times. Before I knew what it was, it really freaked me out and I went to the ER twice in a month or two. It's right under the breastbone. One thing that's helped a little is learning to tell when it's "up" above the diaphragm instead of down below it. A video on YouTube called How to Identify and Correct a Hiatal Hernia was informative for both identifying and helping remedy it. Sit back and relax, and take a deep breath. If your chest goes up, it's probably the hernia acting up. If your abdomen goes out, that's where it should be. When mine is acting up, it feels like a rock hard mass just below the breastbone and is very annoying, and gives me severe anxiety/shakiness. It often feels like it's vibrating, even. And when it's really bad, I can feel my heartbeat and aortic artery pumping very strongly, which I assume is the reduced space due to the hernia. I tried the treatment in the video one night lying in bed when the shaking/tightness started up and within just a few minutes, I could relax again. It felt like my stomach had "dropped."
I'm starting in on a process to go off PPIs altogether, because I feel they are making my condition worse, and I want to be able to fix this naturally. Even in the video, they recommend that having someone else do the procedure on you is much more effective, so you might find a chiropractor that practices hiatal hernia massage. And also check out the relation to the ileocecal valve. I had tenderness in that area for years without knowing what it was, and once I started massaging it, it's not tender anymore. I'm just starting on this healing process, but I'm hopeful! Good luck!