Every user has a maximum of 40 votes in a day, but I can only vote 36 times in a day. It might be some sort of bug, but it could also be a punishment of some sort. I don't know what made this happen or why it happened, but an idea or guess would be much appreciated.
It's every day. Maybe it's how frequently I (up)vote questions and answers; I'm sort of naive.
Is this normal?
According to the meta post in the comment to your answer, your 40 votes are limited by a maximum of 30 applied to answers. Once you hit 30 votes to questions, your voting for the day is done, regardless of any remaining votes you could have allocated to questions.
So, in your case when you voted for your 30th answer, you had only voted for 6 questions, for a total of 36 votes. Since you hit the cap on answer votes, your total votes are locked for the day.
