My own OB said that yes you can ovulate without a period though it's not an exact science when you start ovulating again or even if everybody does. I had a friend go into her 8 wk checkup pregnant again. She DID think you culdn't get pregnant while nursing.
I am now 10 wks PP and have yet to menstruate. But taking no chances! LOL
Anyway, congrats! Not quite Irish twins, but close in age non the less!