I have 3 fibroids; two are quite large and I can certainly feel them in my abdomen. Since they do move around some, they can change position and therefore will sometimes hurt and sometimes not be obvious. The larger one will occasionally push against my abdominal muscles and cause some tightness or even strain. That one also pushes on my ribs and stomach, too. The lower one has caused me some extreme sciatic pain that also comes and goes.
Fibroids can and will swell during pregnancy due to the increased hormone levels, so keep an eye on them via ultrasound. If they've swelled and are causing pressure, that could explain the muscle pain.
I've found acupuncture and chiropractic care has helped with the fibroid pain!