Am I scared to have sex?!

Whenever my boyfriend tries to start something I freak out. The other day he grabbed my ass very gentle but firm and I actually liked it but I said stop really quickly and I didnt really want him to stop..anyways that scared him off so I tried to comfort him by stroking him a little but I kept having this weird knotted feeling in my stomach. He tried to help me out by holding my hand and moving it for me but I just felt so awkward and useless and didn't know what to do next. Eventually I discussed this feeling with my bf and he said I might just not be ready for sex. The truth is I have wanted sex for awhile and I'm always horny but if that's the case why do I get that feeling? So confused...