A laid back seatpost can cause you to hyperextend your legs and lower back. It will also force you to have to lean forward too much. As stated, experiment with different riding positions and postures. Also, make sure the seatpost is at a comfortable height for correct leg extension, and don't put the seat back too far,...that definitely causes lower back pain and neck aches. You're probably stretching your arms and upper torso too far out, causing strain.