Well this might make you feel a bit better. I generally do not get a very strong erection just from making out/and or dry sex. It can even be non existant, why? I don't know exactly, it just happens like that sometimes. I have also felt like you in heated moments, wondering, "Can she tell I don't have an erection and think I'm not enjoying this?" Hopefully girls don't think too much like that because that is not the case. Just relax a little, maybe take things slower. If she wants to go further with you, it will happen, if she doesn't, oh well, you'll be fine either way.