What Causes Pain In The Tailbone?

I have had tailbone pain on and off since my son was born (he is five). It usually occurred at night, was sudden and intense, and last maybe up to 10 minutes. But now it comes any hour of the day, regardless of activity level, can last 45 minutes to an hour, can reoccur, and sometimes includes intestinal discomfort. What is going on?! This pain is so intense it feels like passing a huge sudden bowel movement even though that is not the case. I ve had two children now and it is becoming more often in frequency and more painful.