Hi,
most likely given the graft you had (from your own hamstring right?) and the movement you were doing, you tweaked or tore the hamstring from where the graft was taken. as you had hamstring graft, you will be more susceptible to this, especially so early on in your recovery, when things are healing up from the surgery. make sure you tell your physio, and take it easy! i did something similar a couple of times doing things like bending over!
Vicky