The posture as well as the environment could totally be triggers.
It took me awhile to see that some triggers will give me migraine within a few minutes of exposure and others are delayed. Still others arr not triggers alone, but only cummulatively.
I encourage you to talk to the salon and see if there is a different posture you can sit in. Maybe instead of at the table you could have a board across your lap to have your hands lower. Or maybe there is something you cpild bring with you to put on the chair to raise you up. Etc. Experiment and see if something helps.
Alternatively, investigate different salons/spas in the area to see if any have a more comfortable set up already in place.
Good luck!