Why does it hurt to bend my knee after acl surgery?

Its been over a week and I try to bend my knee since my therapist said once I got that down I'm golden. But sometimes theres like a lock that I can't get past. It hurts really bad and sometimes I just can't get past it but when I do I can bend my knee to nearly 100 degrees which is great. Is this normal? Should this "lock" be happening? I've asked my doctor and therapist and they don't think anything is wrong since when I'm around them I usually get past the lock