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