What Causes Swelling In Back Above Incision Of L5-S1 Fusion?

i had an L4-L5 and L5-S1 fusion two and a half months ago and my back is swelling big time above the incision site. CT shows no fluid so the dr says no need for a needle aspiration. since there's no fluid, that means no infection according to the nurse. i have gained 20 lbs since then and have water retention. i am taking iron due to anemia from the surgery and a high platelete count. can iron, lortab, or flexeril cause water retention? i am back to urinating every half hour to an hour.