Did you eat and drink before bed? Were you cold before you got into the bag? I'm not familiar with that bag - did you have your head covered at night? Was there wind blowing through? Were you more cold on top or on bottom?

The clothes shouldn't make that much difference with a bag that's warm enough for the expected temps. But the pad has to work for that range. I've heard the IAC is not the best pad in colder temps. But there are a number of reasons you might have been cold, and probably it was several things combined - maybe the bag isn't warm enough for you, the pad isn't quite warm enough, and you were slightly dehydrated?

In those temps I couldn't have gotten undressed down to shirt and shorts without catching a chill, just in those minutes between changing and climbing in the bag.