Confused about sex...

sometimes when I hVe sex with my boyfriend I get really into it and enjoy it but then other times, when I’m half way through I just stop and I don’t want to do it anymore and i can’t understand why🤦🏼‍♀️it’s really weird I feel really stupid from it... and I also cannot orgasm and I can’t understand why, when I get to that point my body goes soo weird I start to twitch and stuff and then it goes... I can’t understand why