Well, bumping into your ex sucks. Period. Whether you have feelings for them still or not. Just because he gets a little awkward doesn't mean he's not over her. You need to talk and find out what he's thinking. Chances are he's like everyone else and wishes he never had to see her again.. It may not have anything to do with your relationship. Just ask and communicate.
If you talk and this still doesn't stop, don't freak! There's things you can do too to keep your man's focus on you. I'd keep it subtle though. Don't let the conversation stop so you're in that awkward silence. Keep talking! And stay connected physically too. When I feel like I'm being ignored-- it happens-- I just rest my hand on his thigh. Nothing too crazy but he notices and then he's back to me. If you're not comfortable doing that it can be as simple as squeezing(not painfully) his hand or snuggling a little closer.
Make sure you let him know that its awkward for you too. If he doesn't even make an attempt, or its just too much for you. Hun go find a guy that will make you happy then.

good luck