Hi,
It could be that you have a very small, tight hymen. Or it could be something else that is causing blockage ~ a medical professional would have to determine that...
Keep in mind that when the hymen becomes stretched (ruptured) some women experience bleeding or spotting during or after sex ~ that's common after the very first time for many women. Tampons and fingers are smaller than most penises, so it's no surprise that you are having a difficult time. I saw in your other post that you've tried lubricant and foreplay, but had no luck with either. You might be involuntarily having muscle contractions because you have begun to anticipate problems with penetration. Women of all ages can experience this from time to time, but the key is to learn how to relax and enjoy yourself. When you try to attempt having sex again, next time have him go very slowly; you will need to be extremely relaxed, and use as much lube as possible...
If you continue to have issues with penetration and are unable to have intercourse after another attempt, you definitely should make an appointment to be examined by your doctor/gynecologist.
Take Care