period sex dizziness and shaking

So my boyfriend and I just had period sex (unprotected but he didn't finish), but we had to cut it short because I got extremely dizzy and started shaking and could barely walk on my own. He thinks it's his fault. It was an uncomfortable pain and it felt a lot different then if I wasn't on my period. I tried google but nothing came up. I started birth control back in January and has just taken it an hour before. Any idea to what this means?