What can done once you have had surgery for a hitial hernia but have now developed a hematoma?

Tell your surgeon: It depends on the size and location of the hematoma - you definitely should let your surgeon know. If small, you would just watch - it big may need to be drained. Again it depends of size and location.

Call your surgeon: Don't take a chance - get checked out asap. Surgical site hematoma is usually easily managed. Left untreated, it can certainly resolve on its own but... It can also lead to complications such as infection and persistent regional inflammation which can predispose you to a future incisional hernia.