Rebeca,
Yes, your cast should be changed at the next visit. Depending upon how the bone is healing, it may even be possible to go to a patellar tendon bearing (PTB) cast, which would allow your knee to move.
If in the mean time, the cast really becomes too loose around the tibia (not the thigh, that will be loose depending upon your position), or it begins to rub anywhere, you need to contact your surgeon and inform him/her that you are having cast problems.
Good luck. Hope you do well.