My rooster just laid an egg and there's no other chickens in the coop with him
Would you mind posting a picture of your bird?
I theorize that there are cases of mistaken gender in animals. After all, it occurs in humans more often than people think.
The problem comes because there are what's called primary sex characteristics and secondary sex characteristics. Gender is often determined by secondary sex characteristics because in birds those are the ones we can see. In fact, the only true determination of sex is the presence of either male or female primary sex characteristics which means the presence of ovaries (produce ovum) or testes (produce sperm). Even still, the animal kingdom can throw a monkey wrench into the works because there are animals (humans included) born or hatched all the time that have both sets or one of each. They may develop some or all of the secondary sex characteristics of either sex to varying degrees.