If at all possible try increasing the amount of times you pump at work. Also, you might want to try adding oatmeal to your diet (cereal, hot oatmeal for breakfast, cookies, snack bars, etc.), it can help increase milk production naturally and safely. Make sure you're drinking enough water.
Are you sure your pump is working correctly? You might have to replace a valve. What kind of pump are you using? It's possible this pump might just not be right for you... or, some women just don't have any luck with pumps at all.
Lastly, you can try an herb called Fenugreek to increase your supply. However, talk to your doc about it first. You shouldn't use it if you have certain health conditions, and it can cause side effects. http://www.kellymom.com/herbal/milksupply/fenugreek.html
Or, your doctor can also prescribe a medicine that can help increase your milk production.
Here's a link from KellyMom.com about milk supply: http://www.kellymom.com/bf/supply/index.html
And here's another one specifically about decrease in supply while pumping: http://www.kellymom.com/bf/pumping/pumping_decrease.html
As far as periods go... yes this is normal while breastfeeding. You might skip a few periods, or not have any at all until you start weaning. It's really different for every woman.