WARNING:Picture involved

Hello everyone. I am 23 years old and I am currently not on any form of birth control but have a scheduled appointment to see an obgyn soon. I recently has sex with my boyfriend in the morning of march 15 and took plan b several hours later that day. I was supposed to start my period today march 23 but it actually began yesterday the 22. It was all dark brown blood which is unusual. I thought maybe it was implantation bleeding but I could be wrong and it could have just been old lining left over from my last period. I'm not highly educated on this stuff so I'm not too sure. This morning I went to the bathroom and found what is the picture(sorry in advance, it's gross). Is this just a normal clot or could this be a possible miscarriage? I hardly ever get glots on my period and they are never this big. Neither are they present on the second day either when I do get them. Please help me? Thank you.