wat can be causen or wat can help with constapation belly pain,on a 8month baby?

[LaReina82]

ok it seems to be that she has hard time going she stiffens up n screams n crys her belly gets hard as a rock like too if u touch it she screams more we changed formula foods n also gave more juice n water but nothing helps its gotten so bad that her bottom is red raw like a diaper rash but its clearly not a rash she seems to be in really bad pain when she has to go n it dont stop still she do finish bowels r hard maybe once a day alil mushy but thats even rare tryed heated wash clothes on belly casterol oil prunes prune juice nothing helps is there sumthing more seriously possibly going on not sure if fever has anything to do with this though plus she is teething but i guess been since like 3/4months