Implantation bleeding?? Help

2 weeks ago had a heavy period. Was on an off. I didn't think anything of it. Wiped myself 3 days ago an I would see pink discharge as seen on photo an still do (pic took from google). Been having back pain last night. Really bad cramps. Took one test really really faint line. Took another one it was negative. What to expect? First time going through this. I've Ben on my IUD 6 months ago after birth of my baby. An had it before never had any problems till now. Help please. So confused at this point...