So tired

I try to help around the house as much as I can (I'm staying with my parents until I can save up for a down payment on a house). I'm on maternity leave and I feel like I should help out, but it's so tiring. Just getting out of bed or the chair I'm sitting in hurts my back. The only thing that helps is swimming, which I do to work out and cool off. I just don't know how to explain to my dad when he gets impatient with my slowness and lax cleaning skills.