I found two close together bumps on the back of my 16 month olds head. What could it be?

Probably lymph nodes. If they are moveable , pea sized or smaller and don't seem to hurt her, they are probably lymph nodes. Follow up with your pediatrician at next well visit or sooner if they get bigger, pain or fever.
Might be cysts. Depending on the position, they could be occipital glands. Look for a sore on the scalp. Also look for nits, the hall mark of head lice.They are oval in shape and stuck to individual hairs.