Nir,
Your external interrupt flag is cleared upon vectoring to the interrupt address. The original basic Intel 8051 interrupts were set up to do that.Interrupts added later such as PCA interrupts do not. As for going back into the interrupt, you have to make sure that your hardware causiong the interrupt has released the line before you leave the interrupt or it will set the flag again. You will always perform one instruction outside of the interrupt. In other words, after an interrupt routine has finished (RETI instruction or } in C) the 8051 will perform the next instruction at the location stored on the stack before it can go back into the interrupt routine again.
Allen