First things first--have you talked to your doctor or surgeon about your discomfort? I can only offer speculations, but they can work with you in person to determine what is going on.
It sounds to me that you may not be ready yet for much external rotation at the hips. If the hips are tight, the stress of trying to externally rotate further tends to end up in the medially side of the knee, sometimes crushing the medial meniscus. Since this is the site of your operation, I would speculate that too many external rotations may have contributed to your original conditions. You may be more sensitive now to such pressure, either through swelling after your operation (although by now, all swelling should have subsided) or through less cushioning between the tibia and femur. In any case, since you are experiencing strong discomfort there, I would suggest you stop doing Butterfly for a while.
If your intention is to rehab your knee, you may find this article
of interest: Yin for the Knees.
If you are focussing on your hips, then I would suggest you determine what is stopping you: tension or compression? I suspect you have reached your limit of how much you can open your hips and now trying to get more than your body can offer is placing too much stress in your knees. If you are being stopped due to compression, there is no point trying to "open" those hips any further. You have reached your limit. It is nice to keep what you have, but don't try to go further. If Butterfly is putting too much stress into the knees but you want to keep working the hips, try other postures, like Winged Dragon or Swan pose. See how those feel on your knees.
Let us know how it goes.