The question could be asked a half dozen ways in daily speech. Emphasize "I", and it could mean "I'm powerless in the situation". Emphasize "do" and it could express anxiety or bewilderment. Emphasize "supposed" and it could express impatience or annoyance at being criticized. Emphasize "am" and it could mean "Don't tell me what I should have done when the issue first arose; tell me what I should do today, given the latest set of circumstances".
The imperative contexts of "supposed to" are so varied that there is no "default" intonation.