You'll be swollen for a few weeks, but the majority of it will go down in the first 10 days or so.
Your lower back is aching because you are almost definitely hunching over due to the pain and swelling in front. Try to stand up straight (gently!) when you're walking. It will definitely improve with time.
Sleeping on your side should be fine, just be careful not to roll over directly onto your stoma/appliance. I *never* roll onto my stomach in my sleep so I had no issues sleeping on my side, but some people tend to log roll all night so you should be careful of that for now.
dx'ed UC pancolitis 5/12
past meds: asacol hd, VSL#3, apriso, rowasa, xifaxan, 6mp, cortifoam, pentasa, cimzia, canasa, butyrate, flagyl, cipro, prednisone, remicade, methotrexate, cholestyramine, cortenema
current meds: none!
step one: colectomy, end ileo 1/16/13
step two: j-pouch construction, loop ileo 5/1/13
step three: takedown 7/31/13