Can interrupts occur if I enable interrupts & then quickly disable them ?

No. Specifically, when working in assembly you need a small delay between EI & DI. If you insert a NOP command between EI & DI then this allows the interrupt enable command to complete before DI takes effect. Thus allowing all pending interrupts to be serviced before interrupts are disabled. Any pending interrupts will be serviced in the order of their priority and they all will be serviced at once before control is returned to non-interrupt code.

The reason this delay is needed is because the assembly command EI doesn't take effect immediately but DI does. EI takes effect following the instruction that follows it.