There are many possible causes of pain at just a few weeks post-op.
If the pain is near the incision it could be as simple as a pulling sensation coming from internal stitches or adhesions. Every incision can lead to adhesions (scar tissue internally). And as you move in a way that pulls on adhesions or internal stitches.
Again, you are so early in recovery that pain is expected. When moving about you're likely using muscles that had been cut through during the procedure and now straining that area. You can try massaging the area gently yourself to see if you can kind of massage the incision which can help reduce adhesions. Your body is full of nerves so if the incision is near one it could cause pain as you recover until nerves, adhesions, or muscles heals.
An x-ray is normal at various intervals after surgery and unless it shows an abnormality the expectation is you'll continue to heal. To be honest surgeons don't truly understand the level of pain these surgeries can cause. So if he's not adequately treating your pain you'll likely have better luck with a PM doc.
My surgery involved a vertical back incision around 7" long and a horizontal 5" incision a few inches above my right hip. That incision was very painful for 4-6 weeks. It worsened if I laid on that side, got out of bed on that side, wore my hard LSO brace, or did too much activity. In the shower when using a gentle bath sponge and shower gel I'd massage the area to try to reduce internal adhesions and help the area recover.
You have internal stitches and at just a few weeks post-op you can be feeling a tugging or pain as that whole area heals.
So there are a variety of potential causes. I was lucky that my PM doc took over pain mgmt at 3 weeks post-op (which was 2 weeks after hospital discharge as I was in for 1 week). My surgeon didn't prescribe muscle relaxers but the PM doc did and these helped a lot. When in pain I tend to tense up and then muscles hurt, spasm, etc
Be patient and give it time. Expect to feel differing pains, changes in location and type and intensity of pain for the first 1-2 months at least.
I recently, at 6 months post-op, had a new nerve pain in a new area and it's perfectly normal.