Stealthing, anyone experience this?

I’ve heard of stealthing before and it happened to me. I consented to sex with a condom. I was very clear about protected sex and even offered to buy condoms. So point being there should not have been any reason for my partner to think this was ok. Anyhow getting to the point. This was our first time having sex and he was behind me. When we were done I looked down and saw the condom on the floor. I lost it because it wasn’t full and it looked like it’d been tossed. When I asked him about it he said “oh it must’ve slipped off”. I have an IUD so I’m not worried about a pregnancy but I’m terrified of my chances of getting an STD. I flipped out on him. He was so lax about the whole thing, reassured me he doesn’t have anything. I know why men are stupid enough to do this, because “it feels better” but I didn’t tell him he could do that. I’m feeling ashamed and don’t know how I’m going to trust someone to not do this again. I’m getting tested today but it’s only been 3 days.