I forgot to tell you not to allow her to engage in physical activities, like playing with many kids, to avoid possible spleen damage.
Possible diagnosis, leukemia and mono, are causing spleen and liver enlargement (that's why she probably feels abdominal pain or fullness or swelling) and enlarged spleen is very vulnerable. Even, light pressures on spleen in left upper part of abdomen under ribcage, can cause rupture and bleeding of spleen, which can be fatal.
I don't wanna cause you more worries, but, I think it's better to be aware of possible serious complications.
Please, visit a doctor as soon as possible.
I hope what ever it is, it will be manageable.