if お腹が空いた means "I'm hungry",
then what would be the past tense of お腹が空いた since (i think) it's already in the past tense?
The past tense would be:
This would be along the lines of "my stomach was in the state of being empty" or simply "I was hungry."
Additionally, the "た” in お腹がすいた is not showing "past tense" but is actually showing the completion of an action. In this case, the stomach has emptied.
食べた後で部屋を掃除します。This is the "た” which shows completion of an action.
昨日ケーキを５個食べた！ This is the past tense version of "た".
I believe this た is called 完了形 in grammar terms.
As phirru already answered, “I was hungry” is おなかがすいていた. However, this is not the past form of おなかがすいた. Although phirru explained it in his/her answer, let me be more explicit in this regard.
おなかがすいた is the past form of おなかがすく, which means to become hungry. In other words, おなかがすく refers to the change of the state from “not hungry” to “hungry.” For example,
食事をしないとおなかがすく。 (しょくじをしないとおなかがすく。) We become hungry if we do not eat.
This is why “I am hungry” is おなかがすいた; a literal translation would be “I have become hungry.”
おなかがすいている also means to be hungry, and おなかがすいていた is the past form of this. (Unfortunately, I cannot explain the difference in usage between おなかがすいた and おなかがすいている.)