What do I do when I tell the truth but my boss doesn't believe me?

My grandmother had a heart attack, so I called in to work so I could be at the hospital with her. I am her legal guardian and have responsibilities to be there with her. I come into work the next day and my managers are badgering me for documentation that I was at the hospital with her. The doctor will not provide proof that I was there, he said because it is not "technically" my injury they will not excuse me from work. Now I get looks all day at work as they wait for some sort of proof, and I cannot provide this! How do I get them to believe me, they are very inconsiderate.