depression?

Question
hello,
i have 4 guinea pigs they run freely around my yard and have a large hutch wich they go in to at night they are very spoilt and friendly.
just last week 2 of the girls went outside the fence and wouldnt come in so i thought they may came home on their but they didnt return so i went looking for them?
in devastation i found one of them diamond she had died i believe she was attacked by a dog. i brang herhome and berried her. then i went looking for my other one tinkerbell but i couldnt find her i supposed she was dead. so i had no choice but to forget about her.
the next day i went out to feed my pigs and found her hiding under our cubby house she was dehydrated but alive.
i put her in her cage and gave her fresh food and a new water bottle. she has been fine since. but lately i have noticed that she has been sitting away from the other two pigs and sleeping on her own her and diamond were best friends and followed eachother everwhere. tinkerbelle has dried up skin around her eyes and she has a runny nose im not sure what is wrong with her can you help me please before its too late??
thankyou for your help
mikayla boys

Answer
She most likely has an upper respiratory infection, I have the treatment on my website http://woopets.bravehost.com under guinea pig information. Unusual behavior is a sign of illness typically, the other one might have died from the same thing too.
Brittany