Responsibility meaning that specific one, but I am working towards that
You are right. I am not a man right now and responsibilities are too much for me to handle. And if all goes well, I will not have sex with her again. She isn't in love with me, she's just physically attracted
When she said "No, we're fine," she was probably referring to the time of the month it is for her, and saying that (for example) she was expecting her period the following day. It could be too late for Plan B if you had sex "a couple nights ago." You should ask her why she thinks the timing is not likely to produce a pregnancy. Perhaps it will reassure you.
I am also wondering if it would be a better idea to stop having sex with her, if you have "no feelings for her whatsoever," and have anxiety such that you will freak at moisture in the condom, and if she gets pregnant (from the activity you went into of your own free will) you will "feel trapped." This all makes it seem like you're quite a user and quite a blamer. Neither of those things are what a man does, and at 18, that is what you want to be. Possibly it would be a better plan to find a girlfriend that you can love and care about, so you won't be a user of other people. And it would be a good habit to develop accountability for your own actions.
My guess is that if she thinks there is no problem from the timing, there is no problem, and there is nothing to worry about regarding pregnancy. But if she is secretly in love with you and hoping that a pregnancy would bring the two of you closer, she might be wanting to get pregnant. This would be another reason not to keep up the loveless sex, you really don't sound like you want a tie to her.