Why is my hamster squeaking when she's rolling in her ball?

My hamster is quite active so every night I let her roll in her ball, which she obviously really enjoys. She loves to roll into a room in which it can be hard for her to get out, but this normally doesn't faze her. Last night she was in that room and squeaking, so I helped her out but she squeaked and rolled back in. Any idea why she is squeaking? She does on occasion just appear to randomly squeak while out. I also have 3 cats, but my hamster doesn't seem to mind, but would they have anything to do with her squeaking?