Hello there

I got a question and I really hope that you can help, your website has helped me a lot lately:)
So the problem is that I don't really get the difference beetween these two sentences:
- While I was going downstairs I saw her
- While I was going downstairs I've seen her
I understand the difference between Past Simple and Present Perfect but if we don't have a context what should we choose and why? though I know that the right answer is to use Past Simple but why??? We don't have any indicators like yesterday or two days ago.
I can't sleep because of it, it bothers me a lot. Please, help me!

Thank you beforehand:)