It's extremely rare for an epidural itself to cause back pain. Pain from the epidural is usually nerve damage, and it's very unlikely that you, your mum and your cousin all have nerve damage from an epidural.
Pregnancy puts your body through a huge series of changes so you can carry the baby, and then suddenly the baby disappears, and your body has to go back to normal.
It's often difficult especially when you are carrying a LO around all the time.
I had two epidurals (I accidentally pulled the line out of the first one) and no back pain. I did have pelvic pain for a while, but funnily enough it disappeared when I went back to ballet (which is great for posture).
I'd ask for physio or go to see an osteopath, it's likely that your just a little out of alignment after the birth. Yoga/pilates are also supposed to help.
x