Your are most likely calling that UIAlertView not on the main thread. To make it run on the main thread just use the main dispatch queue like so.
dispatch_async(dispatch_get_main_queue(), ^{
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Success" message:@"You're post has been successfully uploaded." delegate:self cancelButtonTitle:@"Dismiss" otherButtonTitles:nil];
[alertView show];
});