Hello Julia, "I had to eat a fly" is as you said "j'ai dû" like in "J'ai dû manger une mouche", but you can also say "Je devais manger une mouche à cause de/pour x raison" when you're talking about a story that happened in the past. But, when you mean "I must have eaten a fly", you can only use "J'ai dû", but in "manger" you do the action, but in "gober", you undergo it, so you'll say "J'ai dû gober une mouche". Now, in french, the difference can be understood depending on the context. Feel free to ask me more questions if you want to know anything else ! :)