Should I be concerned that I am only 30cm at 34 weeks?

Yes. Concerned, but not worried. Some moms seem to consistently carry their babies in a way that makes them measure "size less than dates", so everything can be perfectly fine. However, such a measurement usually prompts a sonogram to confirm that there has been adequate fetal growth and that there is a normal fluid level.
WHAT DID YOUR OB SAY. You need to ask your ob! If you measure small that is a rough estimate and would be an indication to do a good detailed ultrasound which can check everything including the estimate weighjt and it may be normal or not and the amniotic fluid to make sure it is not too little, so please call your OB as u need an ultrasound.