I doubt it has anything to do with the Yoast plugin unless your settings are not optimal. Given the timing, you may have been affected by the Penguin 3.0 update that hit around Thanksgiving. See http://searchengineland.com/holidays-google-breaks-updates-rules-gives-fresh-penguin-updates-210367 for details. You'll need to do a backlink audit to be sure. Use the Panguin tool to compare your Analytics to Google algorithm updates.
Google does sometime change titles in the SERPs when they deem it appropriate, so the customer support response that you got was likely accurate.