By "sick" do you mean nausea? Could it be nausea from pain in your foreskin? Pain in the foreskin is a sign of tearing, which will cause scarring. In that case just go slower and be more gentle. It can take quite a while (months even).
If it's not just from pain in the foreskin and you're stretching with an erection, it might be pressing your cavernosa (inside of the shaft) down into your pelvis compacting things there. Blocking the testes like this could cause nausea very effectively, I bet. Maybe that could be happening? It could also be a sign of infection and inflammation or nerve irritation that you'd want an exam for. It might even be (the beginning of) a hernia (just to guess). If your testicles are also swollen or painful, you definitely want to get that checked out.
Maybe you should stretch it open without an erection if you can. Though I could see that not being easy (considering how sensitive that skin is). Just go slowly and be gentle. Slowly the skin stretches from new growth (just from regularly applying gentle prolonged pressure to the skin).
If indeed your cavernosa is being pressed down into your abdomen, at least that means you'll gain length once you fix your tight foreskin. It doesn't get much luckier than that.