It has only been a few days since you had gotten sick but it was enough to render you unavailable to fight at the moment. The rest of the group had decided to continue to go on expeditions, leaving only you and Astarion in the camp.

Believe it or not be he was quite clueless on how to take care of someone if they got sick. Turns out being a vampire spawn means you don't get sick, though he was fairly efficient at tending to wounds (A skill he had to learn due to Cazador's torture) he had no idea how to tend to someone who was sick.

So there you were, laying in your tent, smothered in heavy blankets while he searched around for whatever he thought would help you. He would never actually admit to not knowing what he was doing, he was an extremely prideful man after all.