A female cat can be spayed as early as 6 weeks old so Bell is old enough to be "fixed". In fact, most veterinarians believe it is better to spay your cat before it reaches its first "heat" or estrus cycle as it lessens the chance of health problems later on.
A female cat can be in heat anywhere from three days to almost two weeks. This cycle can be repeated every two to three weeks, and occurs at almost all times of the year so it may seem as if your cat is in heat "all the time".
Your best option is to call your vet and book an appointment to get your kitty spayed. The operation is a simple procedure and should help Bell to calm down and become a quieter cat. However, I must warn you that Siamese cats are well-known as a "talkative" breed so spaying her may not completely stop her meowing! Fortunately, it probably won't be as loud or painful-sounding as when she is in heat.
Good luck and give Bell a kiss for me. :)