Exercise for pregnancy

Hi i have bloatedness and slower digestion recently, coupled with constipation. I believe part of the reason is due to lack of exercising. Ever since i got pregnant, i hardly exercise. I tried to go for walks but after work im so tired and want to lie down and rest. Before pregnancy i was not very active, only going for 2.4km runs on sat morning or occasional weekdays. Gynae has advised no intense exercise for time being (17 weeks gestation). Im afraid that im being too unhealthy for baby and me as i often lie down and fall asleep after dinner. My stomach seems more flabby than pregnant looking now.