Thanks for the ideas, everyone!
Also, what camera are using? Looks like light has got in through the back as well.
The camera was a Lubitel 166B.
In the last one, light has definitely got in through the back - the circles and numbers are from the roll film backing paper. Put some black tape over the red film counter window.
The film counter window has a cover that I keep closed except when advancing the film to the next frame. I do this with every roll and this mottling has only ever happened with the Arista. It's harder to see in the second picture, but all the shots from those two rolls had markings from the backing. It does seem like light got on the back but I'm not sure how. Unless the camera was working normally and the film itself was degraded perhaps?
Did you forget to wash the developer out before you put the fix in??? I did the other day with some prints and got similar blotches.
I didn't develop it myself. The guys at the place we take our film are usually pretty good, and we've never had any issue except with these two rolls.
I've put a lot of rolls through that Lubitel and have never had any issues with light leaks. I've underexposed pictures and they didn't come out with splotches. And I don't think these were underexposed - it was perfect Sunny 16 conditions and I exposed for those conditions.
It's really only the film that was the variable, so it seems most likely that it was expired or had gotten too hot or damp, as moominsean and Chalky suggested.
Thanks again! I think from now on, I'll just avoid the Arista 120.