My breasts feel empty! Has my milk supply decreased?

Are you direct latching your baby or exclusive pumping? When my no.2 was 1-2 months old he did not want to latch and I was exclusively pumping breastmilk. I realised my supply was getting lesser and lesser and had the empty feeling when baby turns 4 months old (it could be due to I did not pump consistently and longer for each pump). Thus, I decided to fully latch my baby by training him to suck from breasts and finally I can feel my breasts were full and hard again. If you are exclusively pumping, you can try power pumping method, http://www.lowmilksupply.org/powerpumping.shtml