That's interesting. It could have been exacerbated by the travel a bit. Thank you for the extra information. For children with anxiety problems, a lack of a household routine can sometimes be rough. When travel or other unfamiliar events are interspersed, it can make problems worse. However, don't worry! This doesn't mean you shouldn't travel, it just means that implementing a set schedule (even while traveling) will be even more important.
1) Implementing a schedule starts with having a set time for events that occur around the house (if you don't do that already). Dinner, bathtime, bedtime, storytime, and wake-up time should be set so that she knows what to expect. Work daycare and therapy time into that schedule. The less she is surprised by events, the better. When you go on vacation, the same bathtime, bedtime, and storytime, etc should be continued, as possible.
2) There are many therapists in the Houston area - you are lucky to be close to so many services! Here are a few names: Michelle Puster, M.Ed.(NNN) NNN-NNNNext. 351; Judith Norwood Andrews, Ph.D.(NNN) NNN-NNNN or Matthew Whalen(NNN) NNN-NNNN All of the aforementioned therapists specialize in working with children and test for anxiety/learning issues.
3) The type of anxiety exhibited by selective mutism is best characterized as a Social Anxiety, meaning that it isn't felt globally in all situations. It is an anxiety about being around/with unfamiliar people. It is expressed through one of the only things she has control over - her speech. In time she will learn to manage it in better ways and she will be able to tell you when she's scared.
PS - to reiterate: you did nothing wrong. Anxiety is something that is a character trait - one is generally born with a high-strung personality or a laid back one. Your child may just have a more high strung temperament. The benefits to this is that people with higher anxiety tend to be more organized, more motivated, and more productive once they learn to manage it.
Good luck to you and take care,