My friend experienced the same thing. After her daughter was born she was incredibly sensitive and found that after sex she was crying, it got to the point where she didn't have sex as she knew it would be so painful afterwards.
Did you have an epistiomy? If so it may be that the incisions have over-repaired themselves, leaving a raised area within the vagina that can be sore and cause pain during, or after sex.
If this is the case a simple procedure know as 'Fenton's Procedure' may sort out the problem. In the procedure, this scar tissue is cut out and then the edges of the new incision are placed together and re-stitched. It is normally performed on healed tissue so you may have to wait a few months before you can have the procedure performed.
This may not be the problem but your situation does sound very similar to that of my friend's.