Your septic tank is clogged. Not just full (that’s a different beast), but clogged. Solids have formed a dam, or the baffle is blocked.
Just wash your hands four times. Then once more for luck. how to unclog a septic tank yourself
Pro tip: Use a metal probe rod. Poke the ground every 2 feet in a line from the house. When you hit concrete or fiberglass 2-3 feet down—bingo. Do not open the main giant lid. That’s where the swimming pool of horror lives. Instead, locate the smaller inlet baffle lid (closest to the house). Your septic tank is clogged
Now go take a shower—but keep it short. Your bacteria are working. not a plumber’s yacht.
You can fix this yourself for the price of a pizza, not a plumber’s yacht.