Advice needed

Hi guys, I just need a little bit of advice because I'm not sure what's going on at the minute.

For around about 2 weeks I have been constantly spotting. The blood is either pink or brown and there isn't much on my knickers but its definitely there when I wipe after going to the toilet.

I had sex with my partner last night and there was a lot of thick red and brown blood.

I am sexually active and have had the same partner for 4 year and have both been tested for STIs. Sex is sometimes painful but I usually put this down to him being well endowed and my cervix being low at certain times of the month. I have the implant which runs out in May. This is my third implant, I have never had any problems in the 9 years I have had it.

I am due to have a period tomorrow but haven't had any symptoms of my period coming. There is no blood flowing out and onto my knickers and I don't have to wear a pad so the blood seems to be staying inside but there was a lot on him after sex.

I am going to book to see a GP on Monday but does anyone know what could be happening? Is this just spotting caused by my implant? Could I have some sort of damage from having sex recently? Please help.