I'm gonna guess that your rear shocks are shot. H&R's have a 1.3" drop both front and rear. Swifts were designed with 1.3" front and 1.1" rear. That .2" difference may just be a design decision by H&R that rears it's ugly head by wearing out the rears early. This is why I think so. For 2013, Subaru increased the shock stroke (among other things like stiffening the shocks) in the rear by 10mm or .4". My guess is that Swift only dropped the rears 1.1" for that reason, the 2010-2012 shock stroke was already close to the bottom.
You can test the shocks by quickly putting your weight on the rear bumper and then letting go. The shocks should only cycle once (ie get pushed down, spring up above resting point and then settle). If the back of the car goes up again, shocks are shot. However, this simple test may not work if the car is already sitting on the bump stops.
If you replace them, do it with the 2013 parts. They are stiffer and as stated above, have more stroke.