yes you should definatly be feeling bub moving alot at 28 weeks...if bub does slow down movement wise...better to be safe..and head off to the hospital (i did quite afew times this last pregnancy!!) felt like a noob BUT i wanted to make sure my littel boy was OK...
if it makes you feel alot better being able to hear bubs HB then by all means get a doppler (you can hire them too)