Well, pain is a noun so there's no past tense of it. That's like saying what's the past tense of "happy" or what's the past tense of "a watermelon". Instead, you'd need a verb to go with that noun and then put that verb in past tense form.
Past Tense - I felt pain
Present Tense - I am feeling pain
Future Tense - I will feel pain
In the example above "feel" is the verb. You can then turn that verb into past tense, present tense, or future tense. Pain is the noun that goes with that verb.
