I think it's just because your breasts haven't got used to a long session. Anyway, you are just on NB for like 1 week, right? So a little tenderness is normal, I believe.
Just keep on NB and try not to suddenly increase the length of your sessions in the first month. When your breasts get used to the pressure, you can NB for how long you want.
I usually pump, and hold for a while, then release a little (release until I don't feel the pressure but the domes are still on, I don't want to take them off and on and off ;D), then pump again. I find that helps because it seems to make my swelling better then just pump and hold for longer time.
Good luck, hun :x