Pulled/ torn hamstring after ACL surgery?

4 days after my ACL Reconstruction (Hamstring Graft) and Meniscal Repair i started walking without crutches. everything was good. I went swimming, doing some freestyle and focusing on my kicking when I felt a weird tear in the back of my thigh of my operated leg. i heard a crack/pop/tear (in the back of my thigh) and my hamstring started hurting. my knee feels great but i know whats left of my hamstring hurts in a certain are. i didn't do damage to the graft did i? And anyone have any experience ith this kind of thing?