You can eat less than "normal" and still gain weight. Just depends on the fat and sugar content in the foods that you choose to eat.
If eating the required calories as recommended and still losing weight, then this would need to be investigated.
or if you are eating less than normal
As you were ill and not eating properly, this is the reason for losing the weight.
It would be a good idea to keep a food diary to establish if you have a food intolerance to a particular food.
As you are still feeling bruised inside, uncomfortable and tender, you may have inflammation of the bowels. Triggered off by either bacteria or an intolerance to something that you are eating.
The reason we lose weight is when we consume less calories than our body needs. Once you start to eat more calories than your body needs you will start to gain weight.
If you still persist in having an inflamed and uncomfortable tummy, then see your doctor if you are concerned.
Best wishes.