Napakadali maging breathless sa third trimester lalo na kung on bed rest ka, kasi mas mabigat ang baby at ang uterus presses sa diaphragm mo, pero given na high-risk pregnancy ka with low-lying placenta, definitely worth mentioning to your OB sa next checkup para ma-rule out kung may anemia ka o kung normal lang siya sa stage mo. The bed rest itself can make you feel more sluggish and winded kasi hindi ka gaano gumagalaw, pero pag sobrang hingal even at rest, better safe than sorry na sabihin sa doctor, lalo na may risk factors ka na. How are your other vitals looking, and did your OB say anything about iron supplements?