BIG HUGS
I am ever so sorry for the bad news. I am keeping my fingers crossed that your girl is still strong enough to fight off the pneumonia and have a little more reprieve, though. All the best!
Please take the time to read this guide link below. It won't make for easy reading but it takes you step by step through looking after a guinea pig on terminal care and the natural dying process; how you can spot when the body is closing down, what you can do during the dying process and when to pts (euthanise).
The guide specifically addresses all the practical aspects that usually get glossed over or that are dealt with in a too general way but that can really upset you and throw you during that time. A natural death is usually much more physical than you would expect; it rarely is a gentle fading away, sadly. My guide aims to help you understand better what is happening and to be therefore more in control and not to feel as helpless.
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs
Please also be aware that your own grieving process has already started as well; this happens with the shock of the realisation that any time left is limited and not with the death itself. The bad news is ususally the second worst time in the whole process bar the loss itself. There is a lot you can do for yourself as well in order to make the grieving process easier on you by avoiding one or two nasty emotional traps you could get hung up in. We cannot ease your pain but we can help you feeling less lost and overwhelmed.
Human Bereavement: Grieving, Processing and Support Links for Guinea Pig Owners and Their Children (also contains a chapter on terminal care)
You may want to bookmark these links so you have got them available to read and re-read at need. My thoughts are with you at this difficult and anxious time.