Put my 3 1/2 month old to sleep on her back. When I check on her find her on her side or tummy. Should I be worried about SIDS? Been turning her back.

Great Question! . When your baby gains the strength to move away from the back position around 4 month, you don't have to worry about going back in and flipping her back to her back. SIDS risk not zero, but drastically decreasing between now and six months. Bedding still important, no big fluffy stuffed animals or crib bumpers! Thanks for using Healthtap! .
Position issues. For your own long term peace of mind, always put her down on her back. It is rare to have happen this late, but her position changes during the night are not under your control. As kids get this skill set the risk has fallen substantially.I never get up and re-position them at night.