My 3 yr old has a alk phos level of 275 I was told her liver was under stress... what would be causing this at 3 yrs old?

What did your doctor say?

It's really best to ask this question of your doctor, he/she has all the reports, all the labs, all the test results.

We can guess but that's it.

You may never know what caused it, just what you can do to fix it.

Good luck.