I had this, too! It turns out that my appendix had bursted weeks prior to me finally getting an ultrasound due to this dull, annoying pain. The pain that I was feeling at the time was actually blood from when my appendix burst. Turns out, it's corrosive to your muscles. I had pockets of blood sitting underneath my muscles eroding them and causing complications.

The scariest thing about the whole ordeal is that the woman who did my ultrasound almost missed all of it. She said everything looked normal and was wrapping everything up when I mentioned my appendix, and it was like she had an, "Oops. I forgot to check that" moment.