Should I feel guilty for phoning in sick?

I'm a Postgrad student just starting to work out some part-time work hours. The problem is I signed up for too many hours and having just had 3 days of intense uni days I really feel I need a break (the commite to uni is long infortunately and has taken it out of me). I'm starting to get physical symptoms I associate with stress normally and don't want them to get worse. Is this a good enough reason to take one day off? (I'll go back for my other shifts at the weekend)
Should I feel guilty for phoning in sick?
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