What Causes A Missed Period With Negative Pregnancy Test?

I had implantation bleeding around 10 days ago. This consisted of wiping the a smear of pink/red/brown blood. I've missed a period but had a negative pregnancy test. I'm on propylthiouracil. My periods are usually 25 days in length but now I'm 29 days since last period. I'm 40 years old and have only ever had one late period quite a few years ago. Please can you shed some light? Thanks.