Hi, this looks like ITB (IlioTibial Band) irritation, I personally haven't had it after surgery, but it was one of the things that bothered me most before the HR.
I think the incision cuts through (or along) the ITB band and other structures to get to the hip capsule, so it could be a natural reaction to that, or slight nerve sensitivity after surgery.
In my opinion you shouldn't worry too much, 2 weeks is still very early days, but if it persists surely you should contact your surgeon.
Don't worry, your muscle won't atrophy, it probably has atrophied quite a bit before surgery anyway, gradually over time with the onset of arthritis... Now you will recover from it!
What you mention about full flex on your quad, you shouldn't fully flex your quad at 2 weeks!! At least my surgeon and many others (if not all) impose clear restrictions to hip flexion (the 90 degree rule) for a minimum of 6 weeks, and then, only then, start to work on it a bit more.
I haven't forced flexion at all even now, at 6 months post, and recovered most of the natural flexion gradually over time with exercise and daily activities.