So was she seated and you put your hands on her shoulders from a standing position behind her chair. If so, that's a bit over familiar for me but not necessarily a major issue as long as it doesn't become an annoying habit and there are no other creepy changes in the usual relationship.
If you were facing her and put your hands on both shoulders and you're face to face that's properly weird and you need to apologise ASAP and try to avoid a HR issue.
Yes, she was seated and I was stood behind her and touched her shoulders. She was wearing a cardigan thankfully, so my hands weren't on her skin.