Perhaps some more information would be nice:
1: Any image context is missing; how big are these spots? Where are they on the affected negative(s)?
2: What kind of film? Brand, film type, format, expiration date, storage conditions?
3: How where these processed and by whom? What kind of processing/chemistry etc.?
At this point, your guess is as good as mine. Some kind of backing paper imprint problem is a possibility that comes to mind, but obviously only if this happens to be 120 format film.