there are many reasons why she could have been upset
1. adults generally like to be in control of when, where they go and what they do, by locking her in you denied her of this right
2. many people who are usually ok get justifiably scared or anxious when the above right is denied them
3. Although you are her partner, she probably did not expect you to do this, so it will have placed doubts in her mind about you, this is not a nice thing to have, so she may be cross that your action has placed doubts which she now has to deal with.
take it from one with lots of experience of being locked in a small space againgt their will, so many things flash through the mind in the first minute, everything from panic, how long am i going to be left in here, is the captor going to leave me in here indefinitely, what about food, what about drinks, sleep, the kids, will people miss me, right through to showing you your own mortality and ways to try and escape, anger to your captor is a definite reaction to being locked in.
basically you held her hostage for the time she was locked in and her thoughts and feelings will have been the same as any other hostage.
doing this to a person is very wrong, that is why laws put away hostage holders for a very long time.
please do not ever think of doing this again to her, it really is not nice and is on a totally different level than her pretending to be pregnant.
pretending to be pregnant does not take away any of your human rights, locking someone in does!