My daughter has had constipation from almost birth,she has taken laxatives from then and is now 4yrs old and complaining of a bad back all the time,the laxatives no longer work as they did. She can be sat on the toilet for 40 mins crying asking me to help her.when I have a look she has a great big lump in her bum cheek at the top (lower back)my doctor just keeps trying different meds and says she’ll grow out of it,but its been 4years? Her diets normal but when ever she eats she says her belly hurts.im desperate for some advice and very worried as she starts school soon 
I would recommend either insisting on your doctor to do x-ray’s, blood tests, food allergy tests etc… And if they do not want to do them go and find another doctor!!
Just seeing your child go through this pain should have this doctor doing everything they can to figure it out. Even referring you to specialists. I would start now!!
Thanks for your reply poohbear,I have been back and forth from my doctors,and again today came out with more meds and blaming her diet,I can not make her eat anymore fruit,veg,etc then she already does.im looking at changeing doctors.