It is also likely that it's a peroneal issue which would be affected by the angle of your cleats and/or the way you pedal, as well as foot structure.

Another possibility is peroneal nerve irritation, which now I'm thinking is more likely based on the fact that it comes and goes.

I don't know what state you're in, but some states allow you to go to a PT without a prescription and may or may not be covered by your insurance.

IMO time at PT is wasted on ultrasound. There's little scientific evidence for it for treating most conditions and a lot of people do not even use the correct settings to get the response that they want.