One of our users asked a very good question about our card "Я же говорила." But why not сказала?
сказать - is the perfective aspect, which emphasizes the result or the desire to finish something.
Я скажу ему завтра. I will tell him tomorrow.
Он что-то сказал, когда я вошла. He said something when I came in.
говорить - is the imperfective aspect, which emphasizes the process of doing something (so, the
process is important).
Я же говорила! also implies that the person was telling something more than once.
Завтра он будет говорить с директором. He will talk to the manager tomorrow.
Он что-то говорил, когда я вошла. He was saying something when I came in.
So, technically you can use both in the given sentence, but they would have a slightly different meaning.