Telula, if you had cancer would you feel like a slug calling in when you were sick? How about if you had MS, or Lupus, or many other illnesses? Of course you wouldn't! So you shouldn't feel that way because you have fibromyalgia.
I think, because we have to try to prove our illness, that we do feel guilty because we don't look sick. If the people in your office are really nice and they already know that you have fibromyalgia, maybe print out some good explanations about this syndrome. Really educate them and then thank them for their understanding. Let them know that you are actively searching for something that will help with the pain so you are not missing so much work. You enjoy your job and are doing everything you can to pull your weight in the office. This is the only thing I can think of.
But, don't feel guilty. You wouldn't call in sick if you weren't sick. You know that and, obviously, your boss does too or you wouldn't have a job at all! Just do the best you can. No one can expect more than that.