Just finished keflex for cellulitis infection and responded well. Skin no longer hot or red but still a little swollen. Been 11 days, should I worry?

Not necessarily. It take some time for antibiotics to work and the inflammatory reaction to subside. If concerned see the doctor who gave you the antibiotics.
No. Normal for it to still have some edema. You can use some light compression hose over the counter-- should help.
What's the cause? Cellulitis is basically a visible inflammation (itis) of the skin and soft tissues (red, hot, swollen and tender) . It can be caused by many things including sunburn (radiation), trauma (sprained ankle) or poor vein circulation (stasis dermatitis). It need not be caused by bacteria. If you have repeated episodes of this, get to a wound care or vascular specialist, the diagnoses you have is wrong.