"From what, though?" It's your body and only you can make an educated guess. Some things that might scrape, scratch or abrade that area might include fingernails, or the zipper of one's pants, or some foreign object that got into one's underwear (my husband comes in from working outside with sawdust and barkdust on his clothes, for example, and sometimes washing them just distributes little pieces throughout his clothes). It seems a little bit soon after sex for there to be a scab, unless you were scratched during the act and it immediately bled. The condom would have protected you and possibly you wouldn't even have noticed it happened, in all the excitement.