Could it happen that I live without pain for several days if I have fibro and some other days with horrible pain?

Fibro pain. I believe you mean fibromyalgia, which is generalized pain, with sleep difficulty, alteration of sensation and can have good days and bad days.
Yes. Fibroids of the uterus can cause a varying degree of symptoms and severity of those symptoms. Many people don't have any symptoms, but depending on the size and location of fibroids, some people experience very painful cramps and/or pelvic pain or pressure.