Bullying?

[Puddles1999]

So today I heard a loud scream from the Guinea pig room and I run in to see one of our pigs, Minnie turning her head to the side like it had been clearly hurt. As I look closer I see that hair had been pulled out around her eye. I’m not sure who did it as we also have Penelope & mocha. Either way, should I be worried? She seemed like she had been hurt and it broke my heart. I don’t want anything bad to happen to any of them.

 

[Piggies&buns]

Without a bit more background, it wouldn’t be easy for us to say whether it is bullying - for example, it may be a one off incident.
What size is their cage?
How old are they and how long have they been together?
Are they normally ok with each other?