When you call in and you have available option or sick time, or even if you don't, just say you're not going to make it in that day. If they ask you what's wrong be truthful, either it be you're sick, got the runs, or won't be in for personal reasons. Them asking what's wrong isn't a problem, if they press it to get you to come in just repeat you won't be making it in and hang up. If they try to push it after that then you've got grounds for a grievance, but before then it's just harmless management pressure, or in some rare cases concern, that wouldn't stand up in a grievance.