Yes, Lizzy goes to Netherfield to see after her sister Jane because she is concerned. However, in my heart, I believe that she also goes to Netherfield to see Darcy, because she is beginning to have feelings for him, just as he is having feelings for her (1:6). Her trip to is twofold, to find out how her sister's physical well-being is, and (unconsciously) to see how Darcy reacts to her! There are fire-works already between Elizabeth and Darcy!