There are a few things to try if the bag (down) is too warm for the warm night.

0) Open the bag into something like a quilt.

1) Use your boots to lift up the edges of the "quilt" and create air flow.

2) Have arms or legs outside the "quilt", and get them cooling you off. Same with head, maximum cooling of the head area.

These worked OK for me a long time ago, when I had only 5*F warm down mummy bag. For summer use, I eventually bought a very light Trailwise bag - that was a long time ago.

So, it is possible to get a bag too warm for summer. But there are some inexpensive light synthetic summer bags to tide you through the heat season.

Jim