Red blob on slide film

As you can see, I have this red blob on ectachrome taken with a Stereo Realist. It occurred on only about a quarter of the exposures on this latest roll. Has never occurred before on previous rolls. On the exposures that it does show up, it's toward the end of the roll, they're all on adjacent exposures and the blob is always in the same place. And it only occurs on the right lens of the camera. Any ideas?

(Please disregard the washed out nature of the image. It's likely just because when I took a photo of the slide, I didn't pay attention to white balance setting).

It looks like a light leak. Sorry, that's not terribly helpful (and you probably figured that much).

Since it just started, and at the end of the roll, it may be a new problem that came up while shooting - not necessarily a transient problem. You might try sacrificing some B&W film just to see if it occurs through the entire roll, or again, only near the end.

Since it only shows on some exposures, can you remember a delay between shots or a change in carrying case, direction of light, or something similar?

I once had a leak because of deteriorating foam in my SLR - it was on the end where the back door latches.
I noticed it only happened when I had the half-case off, and then, only when there were a few minutes between exposures.
The ones I fired off in rapid succession didn't have enough time before advancing to get a significant leak.

I've a Realist 45, which is different, so I can't really look at it to come up with any better thoughts.

It's a light leak through the film base. The stereo realist door and latch tends to leak around the take-up spool. Try adding some sealing material in that area.