There are quite a few reasons sex might be a bit different after childbirth. Please know you are not alone and there are ways to make it better.
Relationship (with partner)
All of these play a role with your libido too.
Why might there be pain: hormonal changes and low estrogen leading to dryness, discomfort in the area of tear (if one occurred in delivery), low testosterone with breastfeeding.
Working with a pelvic floor PT along with sex therapist or sex counselor and your Ob/gyn are a great start. We also have many colleagues (PAs and Nurse practitioners) who dedicate their time to sexual health!