You'd be surprised how resilient guineas can be! If you can catch your guinea, wrap her firmly in a towel or blanket, and cover her eyes/head, she would probably tolerate any treatment needed just fine. I've had to catch a few of my wilder guineas to treat them for various things. Once they were restrained and couldn't see what was going on, they settled down just fine. I would start by catching the guinea to see if that's what your dealing with, as it may be something entirely different. As far as catching the guinea, if you can do it after dark, you may be more successful and cause less stress overall. (Since they can not see well in the dark, it will probably involve less chasing/stress.) If you catch her, post some pictures and maybe someone can help identify the problem.