You can try burping baby half way, once the gas is released, she may be able to drink more.
When I called the hospital where I gave birth, the nurse scolded me. They said unless in special situation e.g have to work, inverted nipples, I should not express milk. Just feed baby on demand. Because expressing does not have the same stimulation as feeding baby directly. And by expressing you will dry up faster.
May be your baby is not latching on correctly, open her mouth again until she latch on correctly then feed her. Otherwise, she will hurt you. The cream that the hospital gave me is call Kamillosan. Think you should see a doctor and have it checked. Otherwise, it may get worse.
Drink more milk, fish/papaya soup. Also, there is an organic mother's milk which is a herbal tea. You can get it from Mother Care. And rest/sleep whenever you can. Feed baby as often as you can (eg every 2 hours) directly to stimulate your body to produce breast milk.
Yes, you should sterlize the pump after every use. You can also do so by boiling it.
I would suggest you feed her directly becasue 1) reasons stated above 2) save you all these sterlizing work and 3) directly consuming from breast is cleaner, expressing would in some way contaminate the breast milk.
It is not easy and I too encounted many problems. Hope these can help.