Whether or not you also had rhinoplasty or other nasal or sinus surgery, the rationale for sleeping at a moderately upright position, 30 degree elevation from flat, is that it allow blood and other fluids that might accumulate to drain forward and exit the nose.
The little dressing placed under the nostril is there to " catch" the drainage. Better to come out, rather than stay in because if the blood cannot exit, it will seep further into the tissues surrounding the nose and increase the bruising under the skin. Bruising is blood that traveled away from the nose, partly due to a lack of exit.
Also, if flat on your back that first night, the veins tend to fill and that also can translate to increased swelling and bruising.
One night is not bad.
-Robert Kotler,MD, FACS
Web reference: http://www.robertkotlermd.com