Yes You are probably right. It sounds like the tibialis anterior tendon or the extensor digitorum tendon ( the muscle the pullse your toes back and also pulls the ankle back along with the tib ant muscle). or both may have got bound down near the fracture site, or is rubbing against the fracture site or possible a bit of metal. there are sheaths that lubricate these tendons as tehy cross the ankle joint. sometimes they can get irritated or bound down for much the same reason. If it is one of these problems the physio can help mobilise the tendon once you have the cast off.

The other much less likely possibility is that if the fracture site hasn't united and you are moving the fracture site when you contract your muscles. This is much less likely but can't be ruled out as it is a much more serious problem. I suggest you talk it over with your physio or surgeon when you get the cast off.

All the best with your rehab