我吃了午飯！

午飯真好吃了！
所以這很辛辣了！
:)

我吃了午飯！

這頓午餐真好吃！

但是很辣。

我吃完午飯了！or 我吃完午餐了 (午餐 is more common than 午飯, I think)

午餐真好吃了！
所以這很辛辣了！

但是很辣 or 雖然很辣 (depending on your meaning)
:)

午飯真好吃了！
所以這很辛辣了！(????)
:)

Hi Amy,

I can understand your first sentence. One minor suggestion is remove 了 from your sentence because native won't use 了 like that.

As to the sencond sentence, I am not sure what you mean. Do you want to say your lunch is quite spicy?

Hope it helps. :)