This happened to me when LO turned 8 months and when I thought about it, I had actually skipped a pumping session here and there for a few weeks. Also, my LO has been nursing less and more often which I think made my supply a little confused...
I talked to a lactation counselor, who told me to go back to doing what I was doing when LO was 3 months old which I couldn't really remember... So, I decided to add in one more pumping session a day. This got me the extra bit I needed to have enough for two bottles - so I pump twice at work and then once about 3 hours after LO eats before he goes to bed. I also upped my water, gatorade, coconut water intake and I started to eat oatmeal cookies every day. After a week and a half of this, my supply crept back up so that I could drop the third pumping session. Now, I find that I have one week of normal supply, one week of oversupply and then a week or two of undersupply/just enough (because I do have my period back :-(...)
Around this time your LO might also get a growth spurt which caused havoc with my supply for a good week and a half...
Other food tips - more protein, avoid peppermint. Oh! And more sleep/less stress is possible (that one's hard though!)
And about your pump, I would also suggest changing the membranes on your pump in case they are not working as well.How old is your pump and what kind is it? Sometimes local nursing equipment stores can test the suction on your pump. And you can also rent a hospital grade Medela Symphony if you want to figure out if it's your supply or your pump. I have done that a few times and I did have to get a new pump after all. Good luck!