Hey Sarah
First off... CONGRADULATIONS for making it as far as you have. 5 mos is Awesome! Good for you.
But I too have been there. I have a 10 mos daughter. Around 4mos I had the same problem. Remember there are many factors to take in consideration when you nurse/pump. And any one thing could be the reason.
1. Stress - try not to. You're body will do what it needs to do, just try not to think about it too much. I know how difficult it can be to pump at work. Mentally, physically, emotionally its difficult. You feel like your taking "too much"time, people may be tlaking about you, the room is too hot/cold. You're stressing about all the things you can not change. Just relax, take your time, and focus your thoughts on your daughter. Take a piece of her clothing that she wore the day before to work to lay across your shoulder if you need to. This allows you to have her scent near you.
2. Drink LOTS of Fluids!! Water, juice, whatever
3. Remember to Eat! To increase calorie intake.
4. Get plenty of Rest!
I know these laast three can be difficult at times, but its cruicial for your body to produce your milk supply. When you feel like you've drank enough - have another glass. What is a normal amount for you isn't normal when you breast feed. YOu need more. Rest when you can. Nap during the days your off when she naps, go to bed at night after you put her to bed. It really makes a difference. Be sure to eat. We tend to forget or start to think about the extra pounds you want to take off after having her. The weight will come off, you need to eat. Rule of thumb: healthy foods are better than junk, junk foods are better than none.
5. Pump a few extra times when you are home between feeding. It doesn't have to be for very long. Just to get the stimulation to make your brain think "Ok girls, baby needs a larger supply" Just five minutes, or until what your pumping is free flowing.
At work try to pump about the same time you would have nursed her if you were home.
6. Breast Pump may be clogged or not suctioning as well. Be sure to change your filters. Let all your pieces dry between each use. Keep water away and out of the motor or tubes leading up to the motor.
You're doing a great job! Remember you have given your daughter such an amazing head start. If push comes to shove and you feel is just not workingout. You CAN stop pumping at work.
At 6/7 mos I had to stop pumping at work. It killed me, but so much weight came off my shoulders. I stressed so much when I did pump at work, in the last months. So now, I nurse my daughter, as I've done before - BEFORE I go to work, then as soon as I get home from work, and again before bedtime. So she is still getting breastmilk at least 3x's a day when I'm not home. And her grandma or daddy makes her a bottle of formula after she has breakfast & lunch of baby foods and cereal. Then on days off you can always nurse her during each feeding.
Sorry this was SO long. But I do hope it helps. But no matter what, remember you have done an AWESOME job and have given her such an amazing headstart!
Good luck! Take care of you!