During sex, we had been going at it for a good while. I pulled out for a second and went back in, but she was very dry, and I couldn't get back in. What happened? Is this normal?
Humans are mostly made of water, and vaginal fluid is most certainly water-based. This means it can evaporate under normal earth conditions. This is normal. More normal than that is the fact that even when woman are sexually aroused, they may not make enough vaginal lubrication to make prolonged sexual intercourse comfortable. Thankfully human ingenuity has given us the invention of personal lubricant. Applying a good water-based lube like Astroglide or KY can alleviate the dryness and make sex a lot more comfortable for both of you. Moreover using additional lube can reduce the risk of condom rupture.
So it's a good idea for you and your partner to always use a latex condom with water-based lube, even if that condom was lubricated in its packet.
More info on condom use can be found in our article Condom Basics: A User's Manual