The only thing I can think of is that you might need to make sure bub is nice and snug against you. Sometimes if they're not tied close enough, it can put a bit of strain on muscles, you want to carry their weight as close to you as possible.
Otherwise, I suppose it could just be you using new muscles perhaps...you might want to wear bub a little at a time? Not sure on that, but perhaps it's like getting a sore back from a heavy school bag after the summer break...