Since you had unprotected sex, her being pregnant is a possibility. Pulling out as a method to avoid pregnancy is a mere myth. It does not work! You mentioned she is on the pill. Do you know this for a fact? Does she take them faithfully?
It does seem a bit early for any morning sickness, but each woman reacts differently to pregnancy. Has she missed her period? Has she taken a pregnancy test? Based on everything you have told us pregnancy is a possibility, but isn't likely.
If you do not truly love this woman, my suggestion to you is to talk to her. Explain to her that you do not feel the same way towards her, and that her being pregnant is not going to change that. Please do not try to force a relationship to work out of guilt just because a child may be involved. Everyone usually just ends up being miserable on down the road and it ends in a break-up anyway.
On the other hand, also explain to her that if she is indeed pregnant and it is your child, then you will take responsibility for your child. Hopefully that is the case, that you would be willing to take responsibility for the life you created by being a part of the child's life, and paying any child support, or whatnot. This would also include making sure that the mother gets appropriate care during the pregnancy.
If it is not time for her next period yet, I would wait to see if she gets it, or not. If she doesn't, then I would suggest buying a pregnancy test for her, and being there when she takes it so that you can be there to see the results yourself. I do not know your ex girlfriend, so I am in no position to be a judge of her character. Although it may sound crazy, I have heard of cases where a woman was desperate enough to claim being pregnant to hold onto a man, even going as far as having a pregnant friend take a test to "fake" the pregnancy.
However this may turn out for you I wish you the best of luck.