Sad after sex?

Hey you guys! So I've been with my boyfriend for about a year. I'll admit, the first time we had sex I was NOT ready and got pretty sad after. We stopped having sex for a few months and then now we have been having sex again for a while. But for some reason I get really sad after, like REALLY sad. I know it has nothing to do with him, and the sex is always good and we always use protection. But why do I get so sad? Does anyone else experience this?