What does non specific changes in ecg report mean? doctor keep repeating this exact same word when i asked for explanation and didn't told me anything more he says he is not concerned?

Exactly that, non-specific. It means the lines do not look like the textbook "normal" but they also do not look like anything bad. The important part of that is the expert opinion of someone actually looking at you and your EKG. If that person thinks this is all just normal variation and not important, it is probably normal variation and not important.

Nonspecific changes mean the results are not completely normal but the changes are not specific to any disease and they can happen randomly in patients with some underlying problems such as hypertension, advanced age, etc... Sometimes, those non specific changes can be relevant for certain cases but most often we are neglecting and we do not base our diagnosis on those findings.