Hi there lovingluna, and welcome to Scarleteen.
We can't really say for sure why you're experiencing this bleeding, because we're not doctors and this is the internet. Because you did have a period recently, are not taking birth control, and because you're experiencing a really heavy flow, I would recommend that you check in with a healthcare provider about it. In all likelihood it's not something serious, but whenever you have heavy bleeding that's unexpected, it's a good idea to check in with someone about it. Do you have a gynecologist or regular healthcare provider that you can access?
I did want to point out that the pain that you experienced when having intercourse isn't really supposed to be "part of the first time". Our article
One Bloody Mess: Myths and Realities of Bleeding with First Intercourse gives the full breakdown, but essentially, it's a myth that there needs to be the presence of blood and/or pain during the first time that someone's having intercourse. In fact, anytime someone experiences vaginal pain or bleeding during sex, it usually means that there's not enough lubrication, either human-made or synthetic; the sex or stimulation is too fast or too rough; or that that person is not feeling relaxed or aroused enough for that particular activity to feel comfortable.
We're often taught that it's just part of the 'package deal' of the first time that we have sex, but that pain (and really any pain signals that our bodies give us!) are a pretty good sign that we might need to stop and reassess whatever it is that we're doing that's causing that pain. Our pain signals are usually our bodies communicating to us through a very effective system that something's not right.
As for what you said about only using condoms, is that something that you're comfortable with? Was that a discussion that you had with your partner beforehand, or more something that happened in the moment? How are you feeling about that level of risk/protection for pregnancy and/or STIs?