Nobody here can tell you one way or the other unfortunately, but I can tell you that we all have falls, slips, incidents, call them what you will and we all have this fear. However my surgeon has always reassured me by telling me that the graft is not "just any old rubbish spare part". The fixing is very secure and the graft is way stronger initially than any of us believe. If you heard a popping it could be any of a number of things, but as the graft is still not properly incorporated yet, it is highly unlikely you heard it "snap"
Stop stressing yourself and concentrate on doing your rehab protocol according to your surgeon's wishes or whatever it is your physio has you doing. Be guided by your body and simply keep the joint moving as much as you can. If you feel insecure then go back to using the crutches.
When you see your surgeon for a follow-up you need to recount the incident and also explain any changes you have noticed. Then let them examine you and reassure you.
Onwards and upwards. This recovery is not a straight line sprint to getting back to full functionality, it is a marathon that you keep plodding along, sometimes harder and slower than you want, sometimes faster and easier. Set yourself short term realistic goals as milestones and then tick them off as you pass them. It makes the rehab less frustrating in the long run.
Good luck
