I keep leaking from the left breast only.

What can I do to stop this? I breastfed my 1st child for 11 months and I never had this problem. My baby is a month old and I started working again today but the leaking is so bad I soak through my breast pad, my bra, my under shirt and my shirt. I can pump whenever but that's not the problem I randomly get a letdown and there's nothing I can do about it, I just start leaking.