A couple potential issues:
- The base wasn't well compacted and now it's compacting. As long as it stops and it isn't pooling water badly or otherwise causing problems, I would just live with it. If it gets bad or otherwise starts bothering you, yes, you can have a residential paving company come out and use injection to compensate for the sinking.
- How thick is the concrete? 4" thick and you can expect a 12x13ft flag to crack. There's a reason sidewalks typically have a 4-5ft joint spacing.
- Are there any signs of water washing away the base material? If yes, you need to stop that or it likely will continue to get worse.