Bleeding?

Hi hi!

Hope I’m at the right place with my situation.

On the night of Oct 30 to 31, I had unsafe sex (the Halloween party was a bit too fun).
My period was due on Nov 5; it started on Nov 4.
I’m convinced it was a normal period; it was very similar to periods in previous months.
My cycle is very regular (27-29 days).

Now, on Nov 19, I started bleeding out of nowhere (still bleeding today: Nov 20).
On the 19th it wasn’t much, but it still startled me since I’ve never experienced this before.
Today, Nov 20, it’s quite a lot.
It started with brown (old) blood; then also bright red blood; but also 'clots' with very dark red (black) blood.

A friend told me this could be breakthrough bleeding, which many women experience sometimes, but the timing doesn’t sit right with me.
Why exactly now?! Exactly this month?!
In my opinion, it’s too heavy (and too late) for implantation bleeding (though I’m hoping for it )
Maybe an early miscarriage (but how? I just got my period at the start of the month)?
Or a sexually transmitted infection?

I do feel like I have a tense stomach (when I’m in certain positions), I’m very tired (more tired than usual), and I get out of breath easily.
However, I don’t have any other pregnancy symptoms. Maybe the stuff mentioned above is just in my head rather than being related to anything physical...

I’m in the dark, because I just got my period at the start of this month.

What do you think?

Kind regards,

JS (19 years old)