Hi
If the red urine returns, it is either not an infection or it is a non-bacterial urinary tract infection (sterile cystitis). It can be temporarily suppressed by an antibiotic in its milder to medium forms (most piggies have some faecal bacteria in their urinary tract, seeing that the tiny scent gland is located right in front of the genitalia) but sterile cystitis cannot be cured by it and will come back up.
Or it could be an altogether different issue that could be located in the gut if you are seeing sheer blood drops and not reddish urine and stones anywhere in the urinary tract have been excluded.
You can find more information on sterile cystitis (which is now the most common urinary tract infection in pet guinea pigs) in this link here:
Wiebke's Guide to Pees and Stones
I assume that your vet has palpitated (felt) the bladder for potential stones on your previous visit and has already excluded this aspect? They may want to make sure that there are also no stones in any part from the kidneys downwards.
Please contact the clinic again. All the best.