What's causing soreness in front upper thigh?
It's very difficult  to diagnose this kind of pain without being far more directly involved in the situation. 

However, the immediate issues that come to mind are:


Check the level positioning of the seat. It is possible that either due to ineptitude or lack of care, when the saddle was mounted to the new post, it was not mounted in a position properly level to the ground. In some situations it may be acceptable to have the seat tilted slightly nose down, but most saddles are designed with the idea that they will be level to the ground when the bike is sitting on it's wheels.
Grab the saddle nose with one hand, and the aft of the saddle with the other. Does it rotate side to side more than 2 or 3 millimeters? That kind of movement can cause friction in unusual places. See to it that the tension on the seatpost is adjusted to minimize the play, or replaced with a better quality post, if adjustment is not an option.
Did you also change the saddle at the same time you changed the post? A wider saddle nose, or a wider seat cushion on a saddle can cause friction in the thigh area as well. If the saddle was not changed, and you didn't have these issues prior to the seatpost change, this isn't likely. But worth mentioning.
Make sure the saddle nose is straight in line with the top tube of the frame. If it is misaligned, it can cause friction or pain. 
If none of the are the case, check the seams in your cycling shorts. A bunched or pinched seam can cause that as well, but then it would have nothing to do with the seatpost. 


Let me know if that helps, or try to give more detailed information, and I will do my best to help.