Geralt had always been in tune with his body. Having mutations that made him inhumanly strong, he was careful of his surroundings and aware that a small force from him could mean crushing someone else's bones. He learned quickly how to respond to the first inklings of hunger and the smallest hints of thirst. He knew how much his body could take and where it was okay to be stabbed for at least a few hours. He wasn't unaware of others' bodies either.
He quickly noticed the faint irritation in your tone, the slight hunch of your shoulders, your doubled-over posture, and your constant digging of your knuckles into your stomach. Geralt was worried at first that you had an injury you were keeping from him, but he smelled the blood before he got too worried. Just a natural part of living on the road.
At first, he'd decided to leave you to take care of it yourself—you'd made it this far in your life without someone coddling you. Then you two had turned in at an inn, and you'd started groaning and squirming around in the bed, telling him all about the pain and trying to soldier through it. Damn you and your stubbornness. "That's all I'll need, thank you," he grumbled at the innkeeper, taking the overpriced pot and the kettle in his hands and turning to go back upstairs and try to fix your writhing.
When he walked into the room, he sat on the edge of the bed next to your legs, gently setting the pot and a cup on the bedside table. He pulled a warm damp rag from the kettle, frowning as the water dripped down his arm, and wrung it out. "I brought you things to help with your cycle," he grunted, pulling the blanket from your face and sitting you up gently. "In the morning, we can get more if you need." He tossed the rag onto your torso and raised his eyebrows. "Good for now?"