Should I be concerned if I am not feeling fetal movements at 28 weeks?

See below. Some mothers do not feel the child move until late into the pregnancy. Your OB doctor should be able to check fetal heart tones and movement. This can be done via ultrasound to get a really good idea of movement. Talk to your OB doctor to express your concerns.
No. You should talk to your doctor. Some women perceive movements less than others. This may be due to the position of the placenta. A more concerning scenario would be if the baby moved a certain amount all the time and suddenly stopped moving. You should talk to your doctor immediately if something like that happened.