Giving birth stretches your vagina and sometimes when the vaginal walls are stretched you can suffer a small tear or your doctor might perform an episiotomy (a surgical incision to facilitate childbirth). This will then need to heal.
Even if it looks fine, a scar can be sensitive. If you resume sexual relations too early, it will hurt! So it's really important to wait for long enough: if you experience pain during sex it can cause mental blocks, reduce arousal and lubrication, and it could take you even longer for your sex life to return to normal.