It's like having an overly warm beanbag chair attached at your lower belly, sloshing and swaying right before your rear legs. At best of times its the size of a beach-ball, but your body produces too much milk for it to remain that size for long, milk rapidly filling and stretching it, causing it to ache as the skin stretches ever more tautly. 

At first you can still walk normally, but the girth of your filling udder makes it more of a waddle, then you have to slow as your normal pace causes the heavy milkbag to sway back and forth, milk sloshing, its momentum and ever increasing mass causing it pull at your middle, almost painfully before you stop to let it settle.

You eventually start to walk again, but you feel something dragging against the tips of your teats. Twisting your torso, you look to find a trail of milk on the ground and the nipples of your udder pressing against the surface. The stimulation makes your insides churn a little as your udder suddenly swells larger and rounder, teats pushed aside as the bottom of your udder comes to rest on the ground. 

You try to take a few steps more, but only manage a few before the weight of your udder grows too much, anchoring you in place. The swelling sack even makes you have to adjust your back legs again, spreading them wider. More and more of the sensitive flesh of your udder rests on the ground, milk leaking from your teats pointing in each direction. 

The pressure of your udder slowly ceases to increase, your body sensing that you cannot take more, but you are left standing, the sloshing taut beanbag-chair-sized udder underneath you pinning you in place, large enough that it is supporting most of your weight, not your legs that are forced at a slightly uncomfortable angle. 

Lowering your front half to the ground, you manage to contort to lay down on your side fairly gently, but the movement still causes the milk you contain to slosh heavily back and forth, your teats leaking thick creamy milk as the sensations blank out any other thoughts. Bending at the waist, you try to reach for the nearest teat, but you can't quite reach it. It appears you will be stuck here for quite some time.