Abdomen pain just under the rib?

It could be muscular (if you really flex your abs, maybe?), it could be constipation because your ileoceccal valve is near there, but --really important--your appendix is there too. A ruptured appendix is very serious & needs to be treated by a doctor immediately because otherwise the toxins could lead to sepsis & be fatal. So if you have nausea or fever or diarrhea with that, or severe pain, see a doctor immediately.