intimacy issues

Last night my boyfriend and I were having sex in his truck--were 20/21 and both live at home still so no bed--- but we went on a trip together so we've been in a bed---- anyways. I've have a couple partners and they've all varried in the width and length department so I don't discriminate. I love my boyfriend to pieces and I honestly want to marry him one day. And last night was the first time I had the issue. It's the first time he hasn't gotten me off and I couldn't even get myself off by helping.. but it just felt like I was too lose or he was smaller than what my lady bits like because I'm used to wider and I usually like a little bit of stretch pain down there bc it feels good. 

What do I do about this...? I love him and I'm hopeing it was just an off night because I really want to marry him. And I mean the sex as been hot and heavy since the beginning and it's only been a month since we started having sex so is the hot and heavy sex dying down..? Why ? 

Help a fellow lady out please