If your friend meant to say that she was feeling sad because you weren't with her, she should have written either 'I miss you' or 'I'm missing you' . It would be wrong to say 'I missed you' in this situation. If you and your friend have been separated and then meet again - for example after a holiday - then you could say 'I missed you'. But you can't say this while you are still apart. Is she a native English speaker? If not, I think that maybe she made a mistake.

Always remember that, Use the word I miss you when you are seperated. Use the word I missed you when you are together. Check out my site https://bigben.academy/the-use-of-miss-or-missed-all-differences-explained/. This guide will help you to learn more about "miss" and "missed". Hope it helps :)

Strictly speaking, "missed you" is past (perfect) tense and "miss you" is present tense. Otherwise, they are the same. However, I wouldn't read too much about your friend's intentions into this small distinction.