Guinea pigs have a uterus like every other mammal that hasn't been surgically altered. But although they may have the same body parts as humans, dogs, and other species that bleed during their estrus cycles, bleeding during guinea pig heat is not normal. Therefore, if your guinea pig is bleeding she needs to see an exotics vet as soon as possible.
Then Where Is the Blood From?
It is possible the bleeding could still be from your guinea pig's uterus but if that's the case it is not a normal thing.
She may have a serious problem with her ovaries, uterus, or even her bladder. Infections, tumors, and uroliths (bladder stones) are some issues that can cause bleeding.
Guinea pigs can get a pyometra (which literally means pus in the uterus) just like dogs and cats do and it is caused by a bad infection. Antibiotics are not strong enough to fight off that bad of an infection so emergency surgery to spay your guinea pig is the only cure.
Bladder stones, also known as uroliths, are also common in cavies. Bladder stones are caused when microscopic crystals in the bladder clump together to form a stone. These crystals are formed from changes in the pH of the urine and some research suggests that a diet high in calcium is also at fault. Surgery is the only alternative to removes stones too large to pass through the urethra. Your exotics vet will rule out bladder stones by taking radiographs.
Urinary Tract Infection
An infection in the urinary tract and cystitis (inflammation of the bladder) can cause blood to pass in the urine. You may see blood tinged bedding or drips when she isn't trying to urinate that you may think is from the uterus but it could be from her urine instead. If your cavy will urinate on a non-porous surface, such as a hard kitchen floor or empty plastic litterbox, try to collect the urine with a syringe or pipette to bring to your exotics vet.
They will test the urine for an infection and confirm your guinea pig has a UTI (urinary tract infection) or cystitis.
A mass or tumor, cancerous or not, on any part of a guinea pig's reproductive tract or urinary tract may cause you to see blood being passed. The mass may be irritating to whatever it is growing off of or it may have started bleeding itself and your guinea pig is just passing the blood that is coming from the tumor itself. If this is the case, surgery is once again what is required to see if the tumor can be removed.
Any time surgery is performed risks are involved. Guinea pigs stress very easily and since they cannot be intubated during surgery it can be difficult to keep an open airway. Always keep the risks in mind and see an experienced exotics vet to operate on your beloved cavy. Experts also recommend bringing your other guinea pigs along with your sick guinea pig to the hospital to help keep her calm.
To avoid the possibility of your guinea pig getting a tumor or infection in her reproductive tract get your guinea pig spayed at about 4-6 months of age. This will also prevent her from getting pregnant after her pelvic bones fuse.
To aid in the prevention of uroliths be sure to feed your guinea pig a balanced and appropriate diet of green veggies, timothy hay or sweet grass, and a small number of pellets alongside adequate fluid intake and Vitamin C. A diet consisting of alfalfa hay in adult cavies and a large number of alfalfa pellets can help uroliths form.