high cpu on query

We upgraded to 10.2 a month ago and everything was running fine until 2 days ago when a query started running very slow in prod. It is selecting data from a view using distinct and does an 'order by', too. It runs ok in a copy of the db which was refreshed just after the upgrade. The difference in the number of rows between the 2 databases isn't that much but the execution plans are very different. The indexes are the same in both. I manually updated the stats for the affected tables in another copy which was refreshed yesterday but the plan didn't change. There is a high percentage of cpu involved in the long running query. 

Questions:
- what can cause a difference in the execution plans besides the stats and indexes? 
- in general what can cause a query to use a lot of cpu?

Thanks.
Joe
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