It takes many months for the callus to calcify; the callus glues the 2 fragments together in the first 4-8 weeks but then that has to solidify, the strength of which is determined by the physical forces applied through the callus. By using it once it's united enough, the use determines how the new bone forms. Until then, it feels dodgy. Likewise, any fracture causes tissue damage around the fracture site leading to formation of primitive scar tissue and this needs time and activity to re-organise itself into normal tissue, in the meantime, it feels tight and constricting.
An xray is the only way of knowing how the union is progressing; anything else is (educated) guesswork. The progress will determine your physio and activity. Some pain or discomfort is normal at this early stage, but note that the fibula is a muscle-bearing bone not a weight-bearing one so don't stress about doing damage to yourself.
(Says he while waiting for tibial bone grafts)
Get xrayed.