If she is a year and a half old, this is definietly not her first heat. She has had several before now.
I understand that funds are tight, but you must get her spayed as soon as you can. Most cities have low cost spay/neuter clinics that offer incredibly affordable spays/neuters. Also, research to see if there is a pit bill rescues or organizations in your area. Many pit organizations offer low cost or free spay/neuter certificates to certain vets/clinics due to the mass overbreeding of pits. As pit owners who care about the future of our dogs, many of our organizations will help you with spaying your dog to keep the population from increasing. Spaying just one dog may not sound like much, but keep in mind that her one litter can spiral into hundreds, if not thousands of dogs when they all breed. The numbers are staggering.
There are also many benefits to spaying your dog. Of course, one is that you will no longer need to clean up after her and make her wear diapers!! That should make the both of you very happy!
Another benefit is that if she happens to be around other dogs while she is in heat, her phermones can cause MAJOR issues with other dogs nearby, especially males (whether intact or not), which can turn into fighting and possibly get your dog involved. Then you would be talking about some MAJOR vet bills.
A major benefit to spaying your girl (other than preventing MORE pit bulls) are medical benefits. Females sometimes tend to be better pets if they do not go into heat every six-to-nine months. Heat cycles bring hormonal changes that can lead to personality changes. Repeated heat cycles may subject their reproductive system to uterine and mammary cancers as they age. Some feamles experience false pregnancies that can be a bother to deal with and uterine infections that can be fatal.
Keeping all that in mind, spaying your girl would be the best thing that you could do for her right now. Please consider borrowing some cash or asking for someone to help you with her. It is your responsibility as a dog owner to take care of her, prevent medical issues the best that you can, and to prevent anymore pit babies.
Best of luck!