Recording from a Microphone?

I've built a makeshift recording studio in a small office in my home and have tried recording audio (voice-overs for videos) using an expensive microphone. I have the pop filter. I have the microphone. I record into a laptop (using sound recorder). But although the recording sounds great, every now and then my speech sounds like I am holding my nose. Then, the problem goes away. It happens each time so I am not sure how to remedy this problem. Is it the Microphone? Perhaps the laptop? Maybe the way that I am holding the microphone to my mouth? Any istance would be appreciated.