I believe this is cheating no matter what she says
Does she think that it is cheating?
No. Because we both said we don't cheat and I trust her. I still think it's cheating because she is lying to me
Sometimes couples cannot agree with what they consider cheating or not, so it is up to each set of couple to decided what boundaries they have. If you think it is cheating it doesn't matter if she doesn't, it is cheating and you will need to work it out.
I know what cheating is. This is cheating.
You are right. If you feel like she is cheating, then it is a problem even if she says it's not. Since you now realize this, hopefully you can talk to her more clearly about the issue.