Hello, had an acl reconstruction with hamstring 1.5 years ago. skied yesterday and today knee is painful going up/down stairs. not swollen. retorn?

Probably not: Without an accident or acute event, then it is not likely retorn. It is more likely some inflammation from the increased activity of skiing.

Likely not..: The only way you can re-tear an ACL is with an injury that you will compare to the first time you tore the ACL. It is not uncommon for someone even at a year out to have some pain after athletics. Remember, you likely have not skied since prior to the surgery so this is a new stress on your knee. If a knee swells it is a bad sign; yours in not so don't worry, but see your MD if it persists.