Johnno, a couple of things.

What kind of things is she mentioning about the ex? It's likely that she's frustrated with what happened in the past relationship and she needs to vent, and feels secure enough to talk to you about it.

It could be that she still doesn't know how to act around you. After 3 weeks I'm not so surprised - just give it some time.

Your confusion is warranted. But be patient, if you really like her then you'll take a little time to see what happens. She's with you now, so don't sorry too much about the ex.