Responses
Because he took the drug, he was told his heat would be irregular. As for why it would happen while pregnant, it is up to the author but pregnancy is a hormonal surge; a majority of women are horny while pregnant with many factors as to why they don't act upon it. Probably what's meant to be outside of the drug screwing that up.
How is he in heat if he's pregnant?? The biology is not biology-ing.