Hi!
Which country are you in and how long has your piggy been on antibiotics? They take at least a day to really start kicking in.
If possible try to contact your vet again or an out-of-hours vet if you have them in your country.
Guinea pigs don't suffer from constipation. If there are no poos then it is either from lack of food over the last couple of days or because the gut has stopped working (GI stasis = no gurgling).
All you can do is try your best but pneumonia is unfortunately one of the killing diseases so you are up against it. The guides will hopefully help you to do what you can do but also to know when you have crossed the point of no return. It is always very difficult in a life or death fight; and it is a rather lonely place to be in.
Try to see whether a bowl of steaming water is easing the breathing a little; if yes try a session on a dry towel in a steamed up shower.
Please take the time to read these guides here; they contain hopefully all the necessary practical information.
Emergency, Crisis and Bridging Care until a Vet Appointment (includes syringe feeding advice)
Digestive Disorders: Diarrhea - Bloat - GI Stasis (No Gut Movement) And Not Eating
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs