Raza, it sounds more like a bird has built a nest in the exhaust vent stack for the fan. You didn’t say where the vent was, but it should be on the roof or through a gable end of your attic.
One place that it might be is just into your attic. This is not a good situation as condensation of water from the shower and other water usage can cause all kinds of problems. It is not supposed to be done this way but some builders have been known to “forget” to run the vent outside. Or perhaps it was just not installed tightly and came apart.
In any case, you need to check further, both in the attic and on the roof to see what is happening. Be careful. Let us know what you find if you have further questions.