We can't necessarily know exactly what happened here, but it is pretty common for people to experience some vaginal bleeding after a partner's been rough or heavy-handed during manual sex or intercourse. Your vaginal tissue is pretty delicate, and rough treatment can cause very small abrasions or tears in that tissue that could result in bleeding. It isn't necessarily a big deal; I'd just wait until the bleeding stops, plus one additional day, before further sexual activity, and make sure your partner's being gentle, checking his hands for sharp nails or calluses, and using additional lubricant as needed during manual sex in the future. That may be enough to keep this from happening again.
The hymen isn't something that just "breaks" in one moment, but tissue that gradually wears away over time, often by things that aren't related to sex at all. Bleeding also often doesn't happen with that wearing away of the hymen, because it is usually so gradual. Since you've been having had intercourse already, you may already have less tissue around than you once did to get torn or injured during manual sex, but it's still possible. It's just less likely than the bleeding being because of being rough and/or not using enough lube. My advice would be the same as above, in that case. We have more information on the hymen (or vaginal corona) here: My Corona: The Anatomy Formerly Known as the Hymen & the Myths That Surround It