Hi, I'm sorry to hear your little one is poorly.
If she where mine, my first port of call would be to determine what the actual problem is and treating accordingly rather than just providing pain relief. The fact she is passing blood from either her urinary or uterine system and is steadily loosing weight, and given the medication is for inflammation/pain relief then realistically I wouldn't expect her to rally round from this and will most likely continue to deteriorate.
Given the vet can feel a "mass" in this area, it would be best to determine what this is. It could be a bladder stone Or, if in her womb, could be a cyst or tumour. An ultrasound can be done consciously on your piggy to determine what is going on and then puts you in a better position to move forward.
Obviously there's a high chance that the problem would be something that would require surgery to correct, granted the vet felt this was a feesable option with your individual piggy. Surgery is massive for any guinea pig, especially elderly ones, but with a good piggy savvy vet and a possible cure with surgery this may be something to consider granted she is currently in a position where she's unlikely to recover spontaneously. My 8 year old sow was spayed due to problems with her ovaries and uterus. Fudge was 18 months into a congestive heart failure diagnosis at the time, of which she was given 2 weeks to live as her heart failure was advanced. I managed it with medication and despite a bumpy road she had an excellent quality of life. Her ovaries and uterus where causing bleeding and pain, and although her surgery was going to be massively risky given her heart failure alone never mind her age, i went for it as if she where to pass away with the anaesthetic, it would be painless for her. However if I left her without treatment, she would of slowly and painfully declined before my eyes where I would of had to have her euthanased. Fudge sailed her surgery and lived another 7 months happily and well until her heart gave up the fight and I let her go at 8.5 years old. I don't regret doing the surgery as she had those extra 7 months with a good quality of life, and her demise was unrelated to her surgery.
Please don't think I am trying to sway you into pursuing risky surgery for your piggy- I wouldn't dream of that as I am fully aware every single piggy is different with their needs and coping mechanisms and my situation wasn't the same as yours. However if your vet is willing to carry out a painless, conscious ultrasound on her it should put you into a better position with options rather than not treating her. Of course, I'm not saying that's the wrong thing to do either- sometimes it's best for the individual pet to be managed on pain relief and monitored until a decision needs to be made, no single situation is the same.
I hope your piggy can stay happy and comfortable for as long as possible. Thinking of you x