" Was paying" instead of " was going to pay"

[police]

1) Since I was paying for our lunch, I tried to attract the waiter's attention.

Could you explain to me why we can't use " I was going to pay" instead of " I was paying" in this sentence? Does it mean" Since I intended to pay for lunch, I tried to attract the waiter's attention"?. Please help me out with this. Thanks in advance.

[Alan]

Actually, both are possible.