It sounds like you are experiencing a phenomenon known as the proximity effect. It’s normal behavior for most types of directional mics. If you understand this effect you can use it to your advantage. The ability to work a mic is a good way to find and use different sounds.

Most mics simply tend to emphasize the lower frequencies present in a sound as it is brought closer to the source of that sound. This effect is most pronounced with cardioid or bi-directional mics. Because of their design, omni-directional mics don’t have a problem with this phenomenon.

Without going into too much detail, the effect is basically related to the difference in sound pressure between the front and back of the microphone diaphragm. The largest pressure differential is provided by bass frequencies. This difference in sound pressure is mainly caused by a small delay between when those bass frequencies actually hit the front and back of the diaphragm. This phenomenon causes an interaction between these slightly delayed sounds, emphasizing the lower frequencies of the sound source.

The end result is: The closer your mic is to the sound source, the more the bass frequencies are emphasized.  This is important information to remember when placing your mics.