Welcome to healthcare magic!
After a childbirth there is a phenomenal change in the body of women.
There is lots of hormonal
storming during this period and prolactin
is secreted for milk secretion which suppresses sexual desire.
Another reason is when they are engrossed with care of child, by the end of the day they get dead tired and sex itself needs a mood and energy that is difficult to manage.
Lastly if you are not sharing the load of childcare with her then she might feel as being overburdened and may be blaming you at subconscious level for that.
While you are thinking about sex during bed time she is busy thinking how to manage the night with the baby so that she can have rest to manage the day ahead.
Try to share the responsibility of the childcare and give her three - four hours of uninterrupted rest, plan a weekend eat-out to cut down on the boredom of nursing motherhood.
Manage the things like that;
Let her feel that you are a responsible husband and want to share everything there, not only hungry for sex all the time.
The love will resurrect.
Some weight gain is a rule post pregnancy
I hope this advise would be helpful for you.
Still if you have any queries or doubts, please write me back,
I would be happy to solve them.
Wish you a great health!