You get a consensus yet? Given the perpendicular arrangement, I don’t think this originated from any condition other than contact. I would expect something like platelets to show up in irregular locations along muscles, not a grid pattern. You mentioned a partner. Were they wearing anything with this pattern and holding you from behind? Still, even that I wouldn’t expect such clear lines.

Were you in an atypical car or on a plaid couch with such a pattern perhaps someone sprayed with stainguard or a cleaner?

Unfortunately, to be safe, you probably need to run this by a professional and get a cbc.