What Causes Weakness, Dizziness And Numb Arm During Pregnancy?

I am 35 weeks pregnant and I work out at least 30 min a day. Most days, 60 minutes, 3 miles cardio; excluding Saturday and Sundays. Often, my left arm goes numb. I don t work out past 142 heartbeats. I don t know why but I get weak and dizzy sometimes and frequently tired as work out... I eat enough and drink enough water and sugar.