husband doesn't understand

This is our first child, and I'm just over 4 weeks but find myself getting tired more quickly already. Unfortunately, my husband doesn't understand everything I'm going through and just gets frustrated with me while we are doing outdoor manual labor (building raised garden beds) in 80 degree heat. Ugh, anyone have any recommendations on how to get him to more fully understand all of these changes my body is going through? Although, nothing is visible yet I still feel a big difference in my activity levels!