I have had this happen in Hotel spas, the in-ground my parents had decades ago which was attached to the swimming pool, and one or two of the used tubs I wet tested. Mostly the 'whirlpool' jet, volcano jet or similar.
But I have never had it happen in a HotSpring Portable Spa. I believe that if a jet system is designed correctly it will not only not beat you to a pulp, but it will not cause that annoying itch that some tubs inflict upon you. You can call me silly - Bill does - but I think a spa should make you feel better, not worse. And if I take a few weeks off from my spa, I don't want to have to go through something like you describe the first time I use it again.
But OK - you have the spa you have. Now what do you do? You may want to try turning the jets down - less air, or less water pressure and more air, try different things until you get a good massage without the pain. Temp may have something to do with it as well - try lower first. If you don't have jets which move, then you may need to shift around so that the jet doesn't stay in one place for a long time.
Finally - Hydration. Muscles need to cleanse themselves of waste - saunas and spas are both good for this- so help your body do this by making certain to take in plenty of water before during and after a soak.
HTH
8-)
