I don't know how a doctor can confirm you aren't going to grow anymore at 17. Maybe that's just me.
No, I don't believe you can shrink at your age.
It's common for older people to become a little shorter over time. It isn't dramatic or sudden. It takes place over years and may add up to only an inch or so off of their adult height.
As people get older(process as early as 30, but mostly older), they generally lose some muscle and fat from their bodies as part of the natural aging process. Gravity takes hold, and the cushions between the bones in the spine(disks), get compressed over time.
At your age you don't have to worry about shrinking, you're only 17! The only thing I can think of in the measurment area, is maybe your posture. Make sure that your standing straight up with your back against the wall.
Hope this helped.