What Causes Vaginal Spotting Post Removal Of Nuvaring?

Hi, I had a nuvaring in for around three weeks and I was constantly bleeding so I took it out. After taking it out my husband and i were still sexually active for about a week and I stopped bleeding completely. About a week and a half after taking it out I spotted a small amount of blood and haven t bleed since (it s been three days since and still no blood) I thought this might be implantation bleeding but tested negative today. Any idea of what it could be?