Certified GKH Gardening Expert
It would seem that your grafted citrus has failed. What I see growing, now, is a commonly used rootstock. It is given the name, "Flying Dragon" and does not produce tasty fruit. They will be quite sour and bitter.
This can happen if the top dies, or gets pruned back below the graft point. Once the scion (the grafted top) is lost, it cannot be recovered.
Usually, in container, this can be caused by infection. This appears bacterial. It can happen from remaining too moist for too long, or not having drainage to allow extra moisture out of the container.
Though this will not produce a sweet, or even palatable, fruit it still can be used. If you want to keep it, you will need to treat with a fungicide, first. This article will help:
This article will help you to understand why trees revert back to rootstock:
Here is an article to give you more information on "Flying Dragon" rootstock: