What Causes A Sticky Milky Secretion From Both The Nipples?

I have a sticky yellowy milky secretion from both nipples, but only when the nipples are squeezed. I have a 17 month old girl that I breastfed until she was 8 months old. We have been trying for another baby since August. My last cycle started on 15 Dec and lasted for 4 days. My cycles have been anything between 26 and 36 days, since the middle of 2010. I have very slight breast tenderness and a tingly feeling in my nipples (much like the feeling during let down when breast feeding). What could this secretion be and should I be concerned?