Could I have separated the platysmal muscle 5 months after facelift surgery? Do I need revision or a whole new facelift? (Photo)

the best thing would be to be seen in person by your surgeon who can examine you to determine what is going on.  If this happened 5 months after, you may have a fluid or blood collection that developed.  It is unlikely that a muscle tear would create this appearance.

Based on the photo you provided you'd need to check with your surgeon to make sure you don't have a collection of blood/fluid creating that lump on your left side. It's possible that a suture pulled on the muscle in the area as you were looking up while painting, but that alone wouldn't explain the large swollen area. We're a bit limited in evaluating the area without seeing it in person, though.