How often are you pumping and how often are you nursing?
I don't know about nursing while pregnant, but I do know a lot about pumping because I had a child that wouldn't nurse. I found I had to pump 8-10 times a day at minimum to keep up my normal supply.
Does baby seem satisfied at the breast? How long have you been pumping for and how much milk do you normally get from pumping?
In most cases, the pump isn't as effective as the baby, so mothers will often not get much milk while pumping even though baby is getting plenty at the breast.
I'd suggest doing all the usual things to boost supply - eat well, feed and/or pump as often as possible (when you pump, keep going for a few minutes after the milk seems to stop, to give your body the message you need a bit more), pump in the night if your baby doesn't still night feed, drink plenty of water, and perhaps drink some lactation tea (double check that there's nothing in there you shouldn't consume when pregnant - I don't think there is but different brands are different)
And don't let yourself get stressed. Every drop of breast milk is a bonus to your baby. IF it turns out pregnancy greatly affects your supply, it's not something you can blame yourself for, and your baby will still thrive even if you eventually end up supplementing with formula.
