The reason you got weak and light headed is not because you have given yourself a sever injury but because you saw it, and were thinking about it. the thing to worry about is infection and then the wound healing in a yucky keloid form instead of a nice neat one. Hope you got it stitched back up !
You really need to call the doctor. You could easily get an infection if you are not careful and you don't want that!!
You need to let the doctor know who did the surgery that your wound opened up. You don't want to get an infection in there and he may have to close it back up. Don't delay on notifying them. Take care