Should I really use ‘a’ or ‘her’ before ‘warm hand’ in the sentence?

Should I really use ‘a’ or ‘her’ before ‘warm hand’ in the sentence? If I use ‘her,’ there might be confusion on the part of the reader, who might ask ‘Does it refer to Alyzza or to her aunt?’

Alyzza flinched when her aunt placed (a / her) warm hand on her shoulder.