Advice about calling out

I work as a birthday party coordinator at a local athletic club. I was hired part time (30 hours/week) under the impression that my shifts would only be about 6-8 hours each. My manager (who is 6 years younger than me with minimal obligations) has scheduled me every single weekend in march. The shifts have ranged from 10-12 hours each and they end very late at night. Obviously I'm drained. I'm not working more than my 30/hrs per week, but I am working 20 of them in 2 days, running up and down flights of stairs and all around the club. I average 100 flights of stairs per weekend, so it takes a toll. Then I come home and my job doesn't end because duh- I'm a mom. 

Anyway, I'm scheduled 12-10 today and I'm literally SO SICK. He was sick yesterday and worked, so I know he's going to expect the same from me. I just don't know how I'm going to do it. I need to rest. Draining myself isn't going to make the situation better. Am I an asshole if I call out? How do I even go about it? He knows I'm not feeling well bc we had a conversation about it yesterday and I texted him this morning when I woke up saying I feel horrible. 

Advice please? I  Never call out for myself. Only for the baby if needed.