Hi Rafe -
You probably shouldn't be doing situps yet, maybe ever. Straining the stitches is not a great idea. You can also cause a reherniation by straining too much.
You could have adhesions, but it's also likely the muscles that are hurting are just out of shape. It takes 3 months to completely recover from major surgery, sometimes more for old people like me. (I had 4 C-sections a long time ago and this is much worse.) It depends somewhat on the extent of the surgery. Mine was a long one, 4-5 hours, because of the really big hernia and stomach stuck up in my chest. My muscles in some places are hurting 6 weeks post-op.
I have had no acid since the operation and stopped Nexium cold turkey when I was allowed after the second week. My stomach still tries to reflux (it heaves) when I eat something that is a trigger or eat too much, but it can't come up. I can burp a little, but most of the gas goes down, not up.