Implantation Bleed

So, I have been using an app to monitor my ovulation cycle. It gives you a 6 days fertility window and we were active on day 5 and 6. I have noticed a change in my breasts (slightly bigger and bit tender) and then today I had a very pale pink spot for a few ours. I am wondering if this is an implantation bleed? I have 2 children but never really planned or watched for changes before so I am not very aware of the early stages. I am 40 now and would really love another baby as soon as possible but I am nervous it's not going to happen for some reason. Would be great to hear what you all think. Cheers