Implantation vs breakthrough bleeding

I had sex on the depo shot. No pull out. About 3 weeks after the sex I had dark brown bleeding. Lasting about 5 days straight. Not enough for a tampon. But I had to wear a liner. Then it went away for a day. And came back another 2 before actually leaving. Is this implantation or just breakthrough with the depo? I’ve been on it for 6 months or more now