How in the world did I volunteer for something I didn't know about?

First, a bit of background: I check my work email from home when I'm off. Just in case something comes up that I need to know about before I get back to the office.

The details: My shift ends at 3:30 pm. At that time, I leave work and either go home for a bit and then pick up the kid or head straight to daycare. Either way, I was not aware of the following series of events until the next day when I checked my email. An email was sent Tuesday afternoon at 3:49 pm that 8 volunteers were needed to work on the 4th of July. 4 to work 8-5 and 4 to work 10-7. At 3:52 I got an email that said all the 8-5 shifts were filled, but there was no one signed up yet for 10-7. At 3:55 I got an email from my direct supervisor that he tried to sign me up for 8-5 but they were already filled so I'm down for 10-7. At 3:58, I received an email from the other supervisor who sent the initial email stating that all the shifts were filled. And then finally at 4:17 our manager sends an email thanking the volunteers and saying how great we are. And I had no idea that all this went on until about 11:00 am on Wednesday.

I sent an email at that point to my supervisor who apparently volunteered me to work and I got an out-of-office reply that he's out on vacation until Monday. Great. And I'm afraid to say anything to the other supervisors because I don't want to get my supervisor in trouble for volunteering me without my knowledge or consent. But I really don't want to work that day, much less have to work until 7 pm and then be back at work the next morning at 6:55 am. But if I get a reduced shift based on doctor's orders, then this may all be moot.