Welcome to the IBS board!
Have you noticed that your back pain comes when your monthly cycle does? If so, it is probably a gyno thing. I have a friend that gets horrible headaches and back pain when it is her time of the month. All of us are different... I get horrible cramps and D the first couple days of mine.
It could, however, also be related to IBS. Many people get trapped gas which can radiate pain to all sorts of areas of the body. It could also be attributed to GI spasming. Have you talked to your doc about
these symptoms? Lots of IBSers take antispasmodics. If the heating pad is helping it could definitely be either gas or spasms.