should I feel bad for not wanting to take work shifts when othè people don't show up or call in sick?

I feel guilty sometimes that I don't want to cover other people's shifts. sometimes work calls me less two hours before that time. (And I walk to work if I cant take the bus. and it takes me an hour to walk.) I feel guilty because I know what it's like to work one man short. it's horrible. (I'm a online highschool student so they can throw me shifts any time of day) but.. when they throw me a 6am shift I have to get up at 4, leave at 5 to be there in time. This last week I've had to work extra hours because we were short a man and no one else would cover. I have problems with my feet so standing for the hours I do and walking to and from work with multiple shifts in a row make a it so I don't even want to walk to the kitchen. from my bedroom.