His aunt was worried because he was not his usual self: instead he showed an unusual interest in the medicine.
Tom was an indomitable boy who fancied his own whims and was not to be tamed easily. He was sad because his love Becky Thatcher was not coming to the school for she was not keeping well. The boy was thrown into melancholy and felt dejected. Seeing this, his aunt Polly tried all sorts of remedies and medicines on him. This only made him more irritable and cranky. Tired of all the experiments of his aunt, Tom finally came up with a plan to avoid anymore torture. When he was introduced to the pain killer syrup, he showed extraordinary interest and pretended that he liked it immensely. The aunt was astonished at his behaviour and it was only for her to discover later that the boy was dosing the floor of sitting-room. Then she realised that it was actually a trick that Tom played on her to avoid drinking the pain killer syrup.