Here you have told as per your past and current history you have irritable bowel syndrome, interstitial cystitis, migtain, and fibromylgia. Now recently you have started developing back pain along with muscle spasm feeling in your leg. Here your back pain can be related with multiple issues. It can be due to fibromylgia where it is affecting your spinal muscles, it can be due to IBS where due to overstrain on soft tissue it is causing pain, it can be due to problem at lumbosacral spinal level where there might be issue at lumbar spinal curve or disc related problems disturbing the nerves coming from.that perticular level, it can also be due to interstitial cystitis which is disturbing the soft tissue and creating more inflamation in soft tissue. How ever the conformation can be done by MRI of lumbosacral spine and physical test examination by orthopedic doctor. As per the report and orthopedic examination, ortho doctor will suggest you either surgical correction for severe damage in spine and disc or conservatibe management with physiotherapy treatment and exercises for mild to moderate issues. Here if you have any issue with your digestive system as well for passing motion then you can also check once with gastro entrologist. For back pain you can also use hot and cold pack to have some more relief at home it self. As well do take suggestion on postural correction and physiotherapy spinal exercises properly. This will help to improve strength and stamina of your spine. If your MRI is normal and it gets conform that back pain is due to fibromylgia. I suggest first keep some rest to body and mind, do pranayama and yoga to heal body from inside. Along with this again physiotherapy treatment and suggestion will help you a lot. Also do changes in your life style like keep food habit regular and have simple food with Lora of fruits and fibre, maintain sleeping time etc. Take care.