What you have is too much bass bouncing off the walls. You've created what are called standing waves in your room, which is very common. When there is a standing wave for any frequency, some spots in the room will have almost none of that frequency audible and other spots will have almost double. When you move around, you pass through both kinds of places.
The cheapest workaround is to move your speakers or listening position around until you are hearing the right amount of bass. Aside from being a pain, this create other problems like a very small sweet spot and usually if you fix one frequency this way you make other nearby frequencies worse, so your bass is inconsistent.
The most common solution is to deploy bass traps. These stop the bass from bouncing all around the room and starting standing waves. That way the bass goes from the speakers to your ears and is much more even in level.
The most expensive solution is to use a larger room. The larger the room, the lower the frequencies of standing waves. With a large enough room the frequencies are so low they are not even frequencies we can hear or produce from a speaker, so they don't matter.
This is one of the most common problems in setting up a monitoring system. Do web searches on standing waves and bass traps and you'll find more info than you wish existed on the topic.
Here's a Sound on Sound article on the topic.
And a decent article on monitor placement and treatment.