hussman, “might” is not the past tense of “may.” Might/may are helping verbs that indicate possibility or permission; they don’t stand alone (well, they might, but the main verb is understood):
“Are you going to the store tomorrow?” “I might (go).”
“May I have a cookie?” “Yes, you may (have one).”
Using might/may in the past tense would require a primary verb (which may be understood), e.g.:
“Did you go to the store yesterday?” “I didn’t have any money; otherwise, I might have (gone).”