Congratulations for the new role!
Then, to anwer the question: there is only one foolproof way to find out: talk to them.
There might be many reasons why the behaviour appears to have been changed, and since you mentioned he is the one lobbied for you to get promoted, then it's highly unlikely that he is upset about it now. There are couple of cases which comes to mind (but we cannot tell for sure anyways):
- You were good in your previous role and probably exhibited signs that you are ready to handle some more responsibilities, but maybe the expectation is falling short now, while you are actually handling the extra responsibilities. Anything that appears minor, is not necessarily minor, and may have a larger impact on a bigger picture.
- It may happen that you are repeating the same mistakes. Even if they are minor, repeated mistakes are a strict no-no. Making new mistakes are usually understood.
- Did the promotion change your behaviour / attitude in any way? Do some self-retrospect.
- It maybe just that they are having a bad time (as you have guessed).
Based on the existing level of comfort you have with your manager - try to invite him for a feedback meeting, and gently ask him how you can help him to help you avoid getting into these situations. Take the feedback, acknowledge them and prepare a mitigation / improvement plan. Show them that you're willing to work towards resolving the problem. Then, observe is that is changing the situation or not. You may need couple of iterations (usually not more than that) to find out the root cause of the problem and to solve it.
Best of luck.