I’m recording on a USB mic
Which microphone? If you record using the oblique technique, you can’t be that far off. Half-way between your nose and ear is about the farthest around your head from straight-on you can be before it starts sounding—wait for it—muffled.
You can also get muffling if you’re not speaking into the front of the microphone. We can talk about that when you tell us what the microphone is.
And finally, you can get that if you’re not using your USB microphone at all. Does your computer have a microphone (most laptops do). Are you using it by accident? Start recording and do the scratch test.
Koz