Susanne, if it seems that recovery from this TKR is much more prolonged and difficult than a shoulder or other joint, it's because IT IS!!!! The knee is such a complex joint and many, many things are impacted when the surgeon goes in and messes with bones and soft tissues. The cutting, hammering, and moving things around....not to mention the "test" bending and movement of your leg. I'm telling you, there is a LOT to recover from.
You are doing too much. That's what the pain means. You need a lot more icing and elevation too. And possibly better pain medication. The throbbing is pain and most likely swelling.
Tell me what pain meds you took today when it hurt so bad? And are you on a regular schedule with your meds? If not, you need to be. It's far too early for you to start trying to cut back. You're only at about 5 weeks...VERY early in recovery.
Try not to get discouraged. Things will get better. But it's going to take a while. Progress with a knee replacement comes in weeks and months....not days.
I suggest you limit any exercise to 5 or 10 reps once every other day. Only do what you can do with just the mildest feeling of discomfort. Five minutes on the bike. And with any stretches do not allow them to be done beyond that feeling of mild discomfort. You have plenty of time to get your ROM. Right now, keeping pain and swelling at bay is the most important thing.