If i had a blood clot in my lung after peritoneal cancer surgery, does it mean that something went wrong during my surgery?

No: Postoperative patients are at higher risk for developing blood clots.

No, probably not: You do however, at least, 2 significant risk factors to have blood clot- i.e.- cancer and surgery. Patient with cancer who undergoes major surgery like yours- have a really high risk for development of blood clot- that can go up as high as 40%. Other risk factors also include age, ascites, surgery for > 2 hours, low Albumin level, smoker, cancer burden/presence of metastatic disease etc.