Is my guinea pig dying or just getting old?

We got her when she was pretty young (one of the weaker we saw) but she came out healthy over the years. Lately she's been less active in her cage, but still eats and drinks normally. For the past couple days I've noticed she makes a sound like dove coos, but sometimes a little lower and more grunt-like. It worried me a little since she does that while laying completely still like she's sleeping... I've heard they're less active in the day though, so could she just be old and tired? It worries me because she looks a little out of it maybe. I took her out of the cage & am holding her now, but she's very still and her eyes are more narrow so idk.. she's very relaxed i guess but I just need to know what you think is up. Thank you