Hello,
I too am uncircumcised. What you are experiencing is, from what I know, normal. An uncircumcised penis is supposed to pull back and expose the head during sex. The pain and discomfort is caused by either the skin not being stretched enough or if the skin becomes to tight and cuts of circulation. I still have issues where the head of the penis swells from being cut off. The best solution is to masturbate more often, with lube, an pull back the skin repeatedly. If it hardly ever goes back, which it should for cleaning purposes, you will need to stretch it out.
Hope that helps,
T